My CS1 prof seems to prefer pushing his own IDE (read: he wrote it himself) called JavaRoom.
Anyway, I use Eclipse for a few reasons. First of all, it's really flexible so I can go in and change a whole lot of setting which are then saved to my workspace. Then I can keep this workspace on a USB flash drive or something and use it on my laptop, desktop, friend's desktop, or any of the PCs at school and it'll load up the exact UI settings and the same working environment I was using before. Beyond that, Eclipse has some really nice auto-compillation and auto-build functions that just make life easier. It also dynamically finds syntax errors as you write your statements so you don't compile your classes and find only at the end that you fucked up big time with some 500 syntax errors.