I was in a villa with 11 friends this year for 15 days. The villa was amazing, many trees full of flowers, lemon trees, swimming pool, quiet, 6 bedrooms. But the logistics of dealing with 12 people on holidays in Ibiza is awful. We could never get free tickets for everyone, one day a group wanted to go to Pacha, the other to Amnesia, and some wanted to go to after parties, the others wanted to go back home to sleep and enjoy the next day. Believe me, it's no holidays when you have to plan holidays for 12 people. Even if you say "everyone is on its own", things don't work out this way, there's always someone (or two people) who are responsible for the holidays.
In a villa you have to do your own cleaning (in a hotel, there's room service). There was a staff that was supposed to show every week, but, in 15 days we only saw the cleaning ladies once.
It's a completely different experience to stay in a villa, very fancy, very cool (have your friends over for after parties at the swimming pool), but I would rather use the same amount of money and stay in a better hotel.
For info on villas with a very reliable dealer
www.ibiza-domizile.de.