Well, I updated to iOS 5.0 just a couple days ago and so far so good. It's hard to believe that an OS update of any kind can show considerable improvement in speed when the device hasn't changed at all… as I'm still running the iPhone 4.
I'm very impressed with the updates to the notifications and the camera as a whole but to me the one thing that I love the most is the text-expansion. I'll admit it, I'm not very good at texting and having the ability to create my own shortcuts makes for a much easier time to respond to the few texts I get on a daily basis.
My favorite so far is “amw”… when I type that it will expand to “let me ask my wife”. This gets me out of a lot of things I don't want to do.