The good guys at Android Community dug up a good nugget in their forums, a full-fledged T-Mobile G1 User Review. One of the forum members has spent some good quality time with the G1 and has a few interesting bits to say about it. Namely, it feels like "a sidekick on massive steroids". Also, it has a loud speaker, great call quality, and fairly lighter than he expected.
A quick glance of the pro's and con's show that he's impressed by the slide out keyboard, Android Market, and earpiece but disappointed in the memory leak issues, performance not as smooth as expected, and limited video playback.
Obviously, there are a TON of questions that people have for him, so feel free to hop on over to check out his impromptu Q&A for the T-Mobile G1. We'll be sure to post all the goodies after we analyze them!