Yesterday I went to see an event in a small town near my hometown. I used the iphone gps to get there and to get back (about an hour), music for about 2 hours, about of half an hour of watching videos on youtube, some mail fetching, some web browsing, a few calls and sms, and the battery lasted the whole day. I only got the 20% battery warning right before I got home.
I think that's pretty good. GPS killed my battery quite a bit but I'm going to get a car charger for when I use the iPhone in the car anyway.