Well it depends when abouts in september you go, at the beginning it still will be very busy will all the bars open, even at the end of september it will very busy as it will the end of parties time
Ask the same question in this forum too as there will be a lot of expert advice from hardened club fans

http://www.ibiza-forums.com/

what type of music do they play in the clubs, still house and trance and stuff? also whats the best part of ibiza to go to if you want lots of bars and clubs?