This is not so much about Bagno Castiglione as about Castiglione della Pescaia itself. Seeing as I live here I am sorely tempted to keep the good reviews to myself, but out of loyalty to fellow Tripadvisor travellers I will just say come and see for yourself. Castiglione is a small village (pop 7,000) for most of the year, but a booming seaside resort in July and August (pop 70,000 and up) so choose your timing carefully according to your tastes and budget, because prices skyrocket in high season. In the spring and autumn you can easily rent an apartment for four persons for about 50€ per day, and still benefit from some 15 kilometers of unspoilt beaches, 30,000 hectares of woodland to hike or mountain bike in, a golf course or two and plenty of wind for kite surfing, sailing. Of course, if you're a more peaceful nature you can just relax or go fishing.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.