iOS 5 is pretty nice. Being a beta it is a bit buggy as is to be expected.
I've noticed the new reminders app is particularly bad. Takes forever to open and crashes often both on my iPhone4 and iPad1.
Overall I'm really liking a lot of the new features like notification center/lock screen notifications.
The camera updates are nice (again seems a bit buggy from time to time opening the app)
photo stream via iCloud is really nice. Take a pic on the phone and it shows up almost instantly on the iPad with zero user interaction.
iMessage will be nice once more people are on it I think (similar to BBM)
<edit>I also think the split keyboard on the iPad with be very helpful once I get used to it.
The ability to add mail folders from your device is something I've wanted for a long time and that's great to see.