I had a 3 hour massage/spa session with So Thai a couple of nights ago. The service in this place is world class - it's definaty the best spa place I've ever come across. If you're visiting Patong/Phuket this has to be included as part of your visit. You won't be disappointed. For me, this was one of the top highlights of my visit to Thailand.
I would definately recommend the 3 hour spa session and do this again on my next visit to Thailand.