I was going to hold out on updating to iOS4, but my Spirit jailbroken 3GS went loopy on me a week ago, so I just decided to go ahead and update to the new OS.
My experience is that my battery life isn't as good with iOS4, but it's not horrible either. It's acceptable...how's that? 😀
I've actually been impressed with iOS4, to tell you the truth. I do miss some of my jailbroken apps like LockInfo and SBSettings, but I'm sure I'll jailbreak again when it's released for the new OS. I like the folders that Apple has implemented and the multitasking, while not being perfect, is a nice addition.
Overall, I'm happy with iOS4.