I am reliably informed, however that this is a bit closer to the general impression:
We managed to work out the very hi-tech entrance gates with a little help from one of the attendants and made our way upwards to the rooftop bath for starters.
The water comes out of the ground, I believe, at about 50 degrees. By the time it's in the rooftop pool it's at a pretty constant 35; so nice and toasty, then. It wasn't a crowded day so we found ourselves a nice little corner to bob about in. And that's just about what you do - bob about. Different parts of the pool become jacuzzi-bubbled at different times, so occasionally whilst bobbing about you get a nice tickley feeling around the nethers.
After the better part of an hour we toddled back downstairs to try the Minerva pool in the basement, which is bigger and features gentle currents so you get to travel around a bit whilst bobbing about. Each of the pools also has a massive spigot (great word, spigot) on its edge which spouts spa water onto your shoulders at a pummeling rate; it's lovely.
So can it be recommended as an experience? I should say so. I didn't really expect to think all that much of it, but bobbing about and just chilling (as the young people say) in the warm waters is a delightful thing to do. We came out feeling cleansed, relaxed and mildly prune-like due to having spent two hours mostly submerged. If you've got a bit of spare change (ok, quite a lot of spare change, it's not cheap) and find yourself with a couple of hours in Bath, I'd say you could do a great deal worse than bob about in the Thermae.
A big thank you to Milly (who never reads this stuff, but there you go), we had a great time.