One of my main problems with the G1 is the lack of MS Exchange functionality. I had it on my TyTn and I have it on my Nokia E90, and I love it.
At the moment things are pretty basic, the application only pulls your contacts from Exchange, there is no 2 way synching or anything fancy at all really. However it is good to see that there is active development for this. Once the G1 is publicly available and people have had a few months to play around with it I can image a more functional application will be completed.
One very good point made in the comments of Boy Genius Report is
I know that Android is still too new and has not proven itself as a top notch OS but what gets everyone so excited is that it has promise. Here is a perfect example of the power android has being an open OS. The phone isn’t even out in stores yet and developer are working hard on creating apps for it. This solution is still far from being perfect, but how long did it take the iPhone to get any type of exchange support.