You pretty much get a version of Cydia for ATV (called nitoTV). I really only use it to connect my bluetooth keyboard and of course, for XBMC (I really don't use anything else except for NetFlix and hulu Plus.)
I actually originally hated the iOS 5 update to ATV, but then reverted back to iOS 4, and then back again to iOS 5, as I got used to it.
I also have the fireCore Web browser installed (it's slow as hell, but hey it's a web browser..better than nothing..).
ATV2 with JB, you can install icefilms to stream almost all kinds of movies.