I'd heard about this place and have to say its well worth the trip - for us about 60km / 1and a half hours. From the road it looks spectacular and when you get there even better. There is a smell of sulphur and it does hang around on your bathing gear a bit but not enough to put you off. The whole experience is interesting, invigorating, pleasant and unique. Where can you normally get into a warm bath in the middle of the country, outdoors and just relax. its like being in a nice warm jacuzzi.
True there's not much else here and its fairly basic but its free ! and its really quite enjoyable. I'd recommend it and i'd go back again if in the area. there's a free car park nearby. Do take a towel, bathing gear and some sort of trainers / shoes to get you at least near the water.
We visited Pitigliano on the way here - very picturesque - nice meal there and worth a stop but we didnt find much else there to be honest.