Ibiza is a great place. I've been almost every year since 95.
I'm English and I totally understand most the comments made above.
Basically San Antonio is the cheaper part of Ibiza where you get all the kids under 25 that just go purely to get as drunk as possible the whole time they're out there.
No need to visit San An apart from to check out Cafe Mambo and Del Mar which is next door for the sunsets. You don't get so many drunken kids here as it's a 10 minute walk from San An center aka The West End.
Nice place for a meal while you catch the sunset.
Best clubs depend on what music you like, but best venues are El Divino, Privilege, Amnesia, Space, DC10 & The Underground.
Prices are very expensive, but if you get chatting to the right people it's not to hard to get on the guestlist for most places.
If you can afford it get a villa or pay the extra for a decent hotel.
But if you're just going to party and don't care about accommodation then I'd stay on the outskirts of San An is it's central and easy to get to all the main clubs and bars.
If your buying drugs out there best to get them in the bars or clubs rather than the street dealers as the cops are hot on the case these days.
You might find this site useful
http://www.internationalvillas.net/ibiza_home.html
There's a few lapdancing bars opened up over the last few years which are worth checking out also!
Enjoy