Gotta admit I'm really loving the beta on my 4S. Seems to be rock solid. It is using a fair whack of battery but since I'm docked most of the time it's no great issue.
It's responsive and smooth, very fast to navigate between apps using multitasking, and Control Centre is a godsend having been a jailbreaker just to get SBSettings and NCSettings up till now. The updated Notification Centre is really useful too - I might actually start using it now instead of it just being a place to get to NCSettings. I also love being able to get to functions and notifications from my lock screen.
Every one of my 3rd party apps that I've thrown at it works just fine with no crashes so far. This is after about 6 hours of solid playing around.
By the way - being able to chuck Newsstand in a folder now, and App Store Wishlists .......... FINALLY!!!!!!!!!!
Some of the UI changes still feel "beta" in their conceptualisation. For example the 3G signal dots are fairly large, which when paired with a long carrier name (TelecomNZ) it takes up far too much of the status bar.
By and large it's great though. I'm loving the new concepts and features.