It's open alternate days to men and women (check the website) offering a variety of treatments. The basic package (not a basic price - 43 euros) includes black soap (savon noir), exfoliation (gommage) performed by very strong brillo pad wielding ladies, the use of the steam rooms and a ten minute massage with lavender scented oil. You can upgrade that to twenty minutes or longer.
I had a blissful time yesterday getting really clean and relaxed. It's not often you do nothing except sit and think, or chat quietly to a friend. Top tip is to get there early - as soon as it opens, if you want peace and quiet, as by the middle of the afternoon it was pretty full and noisy. Don't go if you are shy about public nudity (bikini bottoms excepted). Don't wear a bathing suit, you won't get clean enough and anyway no-one is looking.