MTV held their Ibiza party there last year, so my advice is to find some footage and check that out. They had Faithless, Basement Jaxx, Roger Sanchez and Carl Cox to name but a few.
There's a link here with a few video extracts: www.mtv.co.uk/ibiza
Basically, you enter into a large hall or auditorium, more like a concert venue, with a stage at one end. There's also a pool in this room. The rest consists of much smaller annexes off the sides, so you can get a more intimate vibe there too.
Overall, I liked it at the MTV bash, but it was too bloody busy as usual. Although I think the change of venue from a race track to a 4000 capacity club last minute meant they had to crowd people in there. There was definately more than 4000 in there at a time.