Well, it also depends when you are going there!
Summer time can get really rainy, especially on the west coast (Phuket).
Phuket is a very noisy town, prostitution is in your eyes everywhere you go, so not really nice if you are a girl...
Samui and Phi Phi are faboulous for relaxing on the beach and have decent parties at night, without going wild. Lovely places and incredible seafood!