I have noticed massive improvement over 3.0.
Since updating yesterday, my iPhone runs much cooler and is more responsive.
Before the update I would use between 40-60% of my battery every day - and to be honest, I was barely even using it - a few texts, some light surfing and about 2 hours listening to podcasts.
I have put it through its paces today by downloading a 200MB eBook via WiFi, surfing the iTunes store and the app store, making a few long calls and about 1 hour of the iPod. So far I have barely used 30% of my battery.
Paint me happy
Update: Forgot to mention... I'm not jailbroken - just a plain old iPhone 3G.