Noticeably faster boot up, slight improvements in certain apps (youtube, gmail, etc), battery life was better than GB at idle, worse when doing stuff. That was about it for me.
I heard all the problems with ICS, especially with GPS and thought I'd be one of the lucky few, but it took forever to lock on, keyboard was laggy, task manager (holding down home button) took up too much screen space, and the theme was still Samsung's ugliness.
It was something I wanted to try, but in the end I went back to GB.