I'm assuming you hacked, otherwise theres no reason not to do it. You should move to 1.1.1 because of the new features mentioned above, but also because all of the development of 3rd party apps has switched to 1.1.1 now, so there won't be anything new for you. The new installer.app is better organized, and summerboard works better.
Reading the Apple forums there seems to be plenty of reasons not to do it, not the least of which is that many users have had wifi, syncing, and battery life issues since the update. I like some of the new features, but don't have time to fiddle with these issues right now.