Here is a round-up of a bunch of T-Mobile G1 reviews. Overall people seem to like the UI, be unimpressed with the design of the hardware (boring), and complain about the lack of a 3.5mm headphone jack and Bluetooth stereo support.
Some also complain about poor GPS performance, though I never had any problems with that at all. One or two people complained about the hardware buttons being a problem. I agree that they are somewhat stiff, but I never had any real issues with them. Battery life is brought up a few times, and that’s potentially the biggest problem with the G1 in my opinion.
In any case, here are the reviews.