Ok, I went last year with mates (girls and boys) of mid 20s. Stayed at a place in San Antonio bay called The Rosalia. Smallish Hotel but VERY very well run by guy called Nico and his brother. Small pool but clean, just like the rest of the place. San Antonio has Es Paradis, Eden, Cafe Del Mar, Cafe Mambo etc and the infamous "strip" which is basically a street full of bars and mini clubs. Some of these are good and some arent so good, but you will get dragged in to all of them at some point and will soon work out your favourites. The strip is full of pissheads, not really clubbers. Most of the people here stay at the Brisa hotel up the road from Rosalia, which has a bigger pool and most of the occupants run riot around the place all night long.
If you stay in Ibiza Town you have El Divino (posh club, host to Hed Kandi most years), Pacha, Space etc. This is definately the trendier side of Ibiza Id say. Couldnt really tell you about the hotels here.
Amnesia, Privilege are kind of in the middle so your gonna need a taxi but god is it worth it :d
Anything else you wanna know mate?