Trial

One of Europe's smaller nations, Luxemburg, counts just the 1 source of thermal origin, notably in the town of Mondorf-les-Bains.

Located in the south of the country, Mondorf lies on the border with France and seems surprisingly well-kept. Most of the town's economy revolves around what can be described as it's classy thermal pool, it's adjacent park and the consequent guest and entertain facilities.

The hot spring facilities are known as Mondorf Domaine Thermal (Facebook) and this Domaine is located on the side of a hill on the edge of town. 

There's a super large car park which on this sunny September Sunday was packed to the brim. Oh no (expectations heading downhill)!

I had done some pre-soaking research, so I sort of knew roughly what to expect.

Wikipedia shed light on the thermal origins and the broader history of both town and spa:

'It was in the 1840s that the thermal waters were uncovered as a result of deep drilling for salt which had become highly taxed under the Dutch. Karl Gotthelf Kind, who had found salt in Germany and hoped to do the same in Mondorf, discovered the waters after drilling to a record depth of 736 metres. Despite their mineral properties, the waters were not suitable for salt a brownish colour caused by the rich iron content which emerged after distilling. Nevertheless, the local notary, J.-P. Ledure, saw other opportunities for the waters and was successful in finding support for setting up the "Société des Bains de Mondorf". The architect Charles Eydt was immediately commissioned to build the thermal establishment which was inaugurated on 20 June 1847. As a result of the spa's success, the village prospered as rich French guests came to stay in the luxurious hotels which sprang up in the vicinity.

...

Despite acquiring the name of Mondorf-les-Bains on 28 August 1878, the spa had been undergoing a significant decline since 1871. Only after the State took over the facilities on 21 April 1886 were its fortunes improved. Minister of State Paul Eyschen was particularly successful in reviving interest, encouraging visitors to come from Belgium. In the early 20th century, the State invested heavily in the resort adding a pavilion for the original source, a banqueting hall and a reading room as well as the Orangerie and the country’s first indoor swimming pool. The park was also enlarged. A railway to Thionville was opened in 1903 and, in 1913, the Marie-Adelaïde Source, named after the grand duchess was added after drilling to a depth of 464 metres.

...

In 1945, Mondorf's Palace Hotel became Camp Ashcan, a prisoner-of-war camp for senior Nazi dignitaries who awaited trial at Nuremberg. During the allied occupation, that lasted until september 1945, there was no permission to transit in the town from 7 pm to 7 am. The spa continued to prosper in the second half of the 20th century with an outdoor swimming pool (1953), a new thermal centre (1988), and the Casino 2000 which opened in 1983.

The spa welcomes thousands of visitors a year with its richly mineralized waters at 24 °C (75 °F). Set in a park of 36 ha (89 acres), its facilities are among the most modern in Europe. The waters are particularly suitable for the treatment of liver, gastric and respiratory ailments. In addition to a fully equipped fitness pavilion, there are also massage booths, saunas, indoor and outdoor swimming pools, solariums, Turkish baths and whirlpools. Treatments from algae wraps and lava-stone therapy to lymphatic drainage and ayurvedic rituals are said to be especially relaxing'.

Quite a lot to digest.

So they aren't naturally surfacing waters, they were discovered by accident, but their discovery lead to some prosperity as the constant renewal of the bathing facilities has served the local economy well.

The most recent renovation (2015) has lead to it's current set-up.

Advice
My pre-visit research of course touched on the popular review sites.

Google reviews average 4.4 stars (from 1561 reviews). From the past year, an abstract (arbitrary):

'Fantastic facility.

Das Personal, egal welche Abteilung ist super nett, natürlich auch der Kurarzt. Es ist sauber. Was ich als sehr störend empfand, waren die vielen Handybesitzer, die telefoniert haben und auch fotografiert haben. Das soll doch eigentlich ein Ort der Erholung sein. Die Curisten halten sich nicht daran, leise zu sein. Ich schlage ein Handyverbot vor.

Règlement de maillot de bain nul

Je trouve ce lieu sans vie.trop classe peut être?j'étais pas a l'aise

Ça fait du bien!!! La chaleur de l'eau, le calé et la propreté des lieux. Le seul bémol: certains saunas sont "nus".

Tripadvisor rates Mondorf baths with 4 stars. Based on 347 reviews (51 in English, 262 in French, just 13 in German). 

Some snippets from last 12 months:

'Zu teuer, Innenbereich veraltet und teilweise schmutzig
Point négatif les gens nu qui marche avec leur asticot dans les endroits où les asticots sont cacher normalement.
Pour finir, si vous êtes seulement fan de piscine uniquement, l'entrée est relativement chère
Der Innenbereich müsste dringend renoviert werden.
Sehr sauber, ...
Domaine très sympathique
Au top !!
Fabuleux !
Nous devons être nu pour les saunas ce que je trouve tout à fait normal. Dommages par contre de devoir tjs remettre son maillot car si vous souhaitez faire un sauna puis un deuxième, vous devez sans cesse mettre et démettre votre maillot . Du coup je ne le met plus du tout je suis nue sous le peignoir, Cest beaucoup plus simple. L'endroit est très propres! Je recommande cette établissement. Et arrêté de dire c'est honteux les gens sont nus.
Le centre thermal possède une Hygiène et propreté impeccable'.

A lesser known review site, saunafans.de, has an extensive review (2018):

'Insgesamt war die „Domaine Thermal“ mit Abstand die beste Saunaanlage, die ich in Luxemburg besucht habe, wenngleich doch einige Minuspunkte das sehr gute Ergebnis arg eintrüben: So ist das Gebäude der Therme arg veraltet, wenngleich auch gut gepflegt (wobei man natürlich auch nicht alle fünf Jahre alles abreißen und wieder neu aufbauen kann). Aber speziell das Thermalbecken ist arg in die Jahre gekommen, und der Umkleidebereich ist sehr unattraktiv gestaltet. Hinzu kommt der Umstand, daß man ständig, wenn man zwischen den Saunagängen die Wasserbecken nutzen möchte, aus den Badesachen rein und raus muß. Hier sollte Mondorf konsequent Therme und Sauna trennen, auch mit unterschiedlichen Preisen für die verschiedenen Bereiche. Der ohnehin kaum genutzte Innenbereich könnte dann in eine Textilsauna umgewandelt werden. Das Angebot der Bar ist eher bescheiden, wenngleich auch zu moderaten Preisen.

Stichwort Preis: Der ist mit 39 € für die Tageskarte natürlich enorm. (Wenn man dann aber mit der Saarland-Therme, die mittlerweile bis zu 35 € für einen Tagesaufenthalt aufruft, vergleicht, geradezu ein Schnäppchen.) Daher sollte man schon einen Tag für den Aufenthalt einplanen. Auch Arrangements mit dem dazugehörigen Hotel sind selbstverständlich möglich. Wochenend- und Feiertagszuschläge werden auf Tageskarten nicht erhoben, wohl aber auf Stundentarife in Höhe von 2 €. Geöffnet ist täglich ab 9 Uhr, montags bis donnerstags bis 22, freitags bis 23, samstags bis 21 und sonn- und feiertags bis 20 Uhr. Jugendlichen unter 16 Jahren ist der Zutritt zu den Saunen nicht erlaubt. Es herrscht täglich gemischter Betrieb'.

So what to expect: 
- high entrance price. 
- some distaste (confusion even) concerning nudity or what to expect, 
- staff not kind / kind, 
- facilities not clean / clean, etc.

So let's look at the entrance price.  
Mondorf Domaine Thermal note that a full day admission is 40€! But there are cheaper alternatives: a 2 hour sojourn costs half this (19,5€) though there's an additional 2€ charge for weekends; 3 hours is 26,5€.

And let's look at the confusion concerning wearables. Again from the same official site:

'Please bear in mind that a sauna is enjoyed without swimsuit. Just bring two towels along: one to wrap around your waist or armpits, and the other to put under your feet. This basic equipment will enable you to make the most of your session'.

Hmm, still confusing, does one take the sauna naked or not?

Aficionado

From the parking we sauntered to the entrance.
It's not too busy at the reception and after the receipt of a bracelet, we proceed onwards to the locker-rooms. 
This area is hugely confusing: rooms are all over the place but the lockers are all taken, it seems. The changing rooms themselves are very narrow, but there aren't too many people milling around, so changing isn't too difficult.

Onwards we proceed to ..., well somewhere outside. 
All very confusing as more and more locker-rooms become apparent until finally there are signs to showers, these are gender specific. 

From here you can step outside. 
Outside we are presented with a big lido pool, which is quite busy. The water is definitely not thermal (?), the thermal pool is off to the right, slightly lower. 
The thermal pool lies in an area that's available to all age groups. There are some families here but not so many; maybe the entrance price discussion of above bears influence on this? 
The thermal pool is nice and warm. But also busy. There are a number of water-massage stations but with a strict procedure: we seem to have caused sacrilege by not starting at the start, nor pressuring the person before us to move on. Nice.

We return to the area reserved for the older generations (above sixteens) and seek out a sauna. As said I had done my research previous and though management frowns on nakeds walking around and sunning, sauna's are all au-naturel.

The first sauna is gearing up for an Aufguss session, but there's space on the middle bench. Slowly the sauna gets packed as more and more persons are crammed in, though notably many packed in towels. Finally the Aufguss session proceeds and even though I have doubts whether I'll make it to the end, there's no escape possible, simply due to the number of bodies. Surprisingly after the thorough Aufguss, the crowd dissipates, mostly packed in towels and skipping the post shower. Hmmm. 

I hadn't taken my camera with me; a practice often frowned on in European spa's. Below though is a very seldom photo, found on instagram of what essentially is the full domain: in front, the hotter higher up pool, below that the lido-esque adults-only pool, while behind the building in the center is the the main thermal pool. All facilities are on your right, while on your left the few roofs on display are of the various sauna's:

rgb_88.49.197

Fosse sempre così 😎🤩...#summer in #luxembourg #therme #pool #weekendvibes 🥳

During the afternoon the practice of avoiding the naked body is held in high esteem, though as the afternoon proceeds towards the 5 o'clock mark, the number of natural aficionado's seems to rise; and it seems that management is relaxed enough to tolerate natural bodies occurring in the sauna areas above the pools. But nobody dares a skinny dip in the pool itself; apparently that's where the limit lies. 

Other observations:

There was actually quite a lot of activity during the afternoon, as there are many sauna's and Aufguss sessions constantly taking place. 

Alas, many of the lawn recliners seem reserved, clearly there seems to be a disconnect between the number of visitors and recliners. Is it normal to expect more persons share one recliner?

In the higher section above the larger pool, there is a nice hotter pool (foreground photo above), much smaller, which also sees loads of soakers.

While the sun slowly drops, we decide to call it a day. 
The reverse search for showers and locker rooms takes place. 
And after we locate our locker we are forced to wait until others have vacated the locker-room, there's simply not enough space. But also it seems not everybody is in cooperative frame of mind.

Overall we felt that the experience was not worth it's value. 
It was to a certain level quite novel: the dress / undressing part that is. 
Yes, it's real mineral water but the existence of thermal water only plays a supporting role in the whole bathing complex, so for the soak itself; not necessary.
I also believe management could have done a better job to make the whole process much more easy-going and user-friendly

Have I missed anything else noteworthy?
There's more from the Mondorf Domaine Thermal:

'The beneficial effects of authentic thermal waters recognised for more than 160 years and heated at 36 °C are geared essentially to the general relaxation of the muscles, the improvement of blood circulation, and relief from pains and inflammations. The large quantity of mineral salts present in the thermal water produces a state of weightlessness that facilitates movements and relaxes the muscles, particularly during aqua course sessions.

...

Michel LUCIUS thermal spring

Depth : 750 m

Flow rate : 440 litres/minute

pH : 6,8

Total mineralisation : 14 555,05 mg/kg water

Minerals : chlorides, sulphates, hydrogen carbonates, fluoride, iodine, sodium, calcium, magnesium, potassium, iron, manganese'.

The thermal pool itself:

source

Finally, Léssentiel.lu (Jul. 31) reports on healthy profits being made, especially since the 2015 renovation.

Postscript 2022: May we have your votes? Six out of ten then. The hogging of the pool walls, the great empty spaces, the confusion concerning clothing, the packed aufguss. Not much positives ...