My biggest complain would be the bad drain of power on the iPhone 3GS after purchasing it in replacement of my old defunct phone.
Although i love the interface and multitouch, the single task machine seems to be draining the battery like drinking water. Within minutes, you can see the percentage drop 1%, 2%, … The worst i have seen is that it drops 25% overnight doing nothing and the Usage indicator shows 1 hr in use!!!
I flashed in version 3.1.3 firmware and enabled 3G (it is a 3G phone why disable it), disabled WIFI, Notifications and Push Mail. I was able to get a pretty good result.
Head over here and see my results log since 3-Feb-10 1100pm last night till now 4-Feb-10 1034pm