Ibiza's always good mate!
In all seriousness though, if you wanna chill it'd be cool, but there isnt loads going on.
its interesting to see how different it is compared to summer.
Obviously its good round NYE time, but if you're looking to go out etc id say it'd be 99% locals.
Ive not been in December, but have been in late october/november...
Its still relatively warm, and pacha opens at the weekends.