I went in October last year and spent 3 nights in Koh Phangnan for the Pre-Full Moon and Full Moon Parties ...
I gotta say it was the best holiday ever!
We stayed at the Power Beach Resort which was also in Baan Tai ... it's not far at all and you'll want the peace and quiet after partying till the early hours of the morning ... I think we got back around midday after both parties ... It's CRAZY!
If you walk to the far left of Haadrin Beach there is a place called Mushroom Mountain ... it's a bar that sells nice cocktails and other things ...
Like others have said ... just don't get too careless and you'll have a great time!
I'm really jealous .. I really want to go back this year but I think I might give Rio a try instead ...