When using an application: if the phone is out of service and comes back into service, whatever application you're using seems to crash. I take the NYC subway every morning to work, and there's one point where the train crosses the Manhattan bridge where I get signal. Without fail, my phone seems to crash at that point.
Also, does anyone else think this phone/OS is absolutely terrible at memory management? After a day or two of use, the response becomes borderline unusable until I power-cycle. A way to kill applications in the background would be a helpful thing, I think...
I had the first gen iPhone prior to the G1, and I must say that it was much more responsive overall. There were times it would stutter, but overall the touch response was on point. This phone seems to not recognize inputs from the touch screen as readily as the iPhone does. On the other hand, the iPhone doesn't have applications running in the background, which may account for it's responsiveness.
Overall, I'm happy I switched to the G1...I just hate that I need to reset the damn thing so often to keep it responsive at an acceptable level.