I've spent the last three new years on the island. If you've only seen it in the summer you can't imagine how different it is. Pacha always opens for NYE and sometimes Amnesia, DC10 and Space. Lots of smaller places such as Aura and Bambuddha Grove also have parties. The locals tend to get dressed up to spend midnight with their families before they go out. I remember being in the main room at Amnesia with loads of people in suits. Very strange.
It is fun, but I don't know if I'd spend the money to get to the island at that time of year. There aren't any direct flights and getting to Palma or Barcelona isn't cheap. On the other hand, it's a great time to see Ibiza. You can get warm days in the 20s and the first wild flowers of spring are just appearing.