my friend just got back from koh phi phi a week ago, it's a quick(ish) boat ride from phuket. anyway she said it was great, not too crowded, i saw the pictures and they look amazing. it's not party central but there's definitely bars and a few people about, and she thinks there's probably a disco somewhere around the main drag as well (she wasn't really in party mode when she was there) i'd suggested the place to her sight unseen cause i kept hearing great reviews...

you might check "the thorn tree" on the lonely planet's website if you haven't already, i'm sure this question comes up a lot...