I'm beginning to think I have a bad battery because my battery performance is not as good as my 1st Gen iPhone. Sure, on the first day I was all excited about the phone and used all the features and such, but it quickly became just my phone. I'm totally back to using my phone like I've always used it and I am constantly finding my battery at below 50% after a daytime of little use. It's not so bad that it's causing me to be without a phone, but it's definitely not lasting as long as my 1st Gen.
Same with me. I have 3G on, fetch data 15 min, push notifications on, wifi on, and 40% screen brightness. This is only my 2nd day of owning the 3GS and right now I have about 9 hours stand by, 4 hours of usage (usage was one 30 min vid, listening to music for 30min, and light browsing/checking email. Now I have 23% battery remaining. I hope this poor battery performance gets fixed though in an update. Even with 3.0 installed, my first gen iPhone lasted MUCH longer.