Simply useful phone.
I'm very happy with my G1. I pre-ordered it from TMO last October and have been happy with it ever since. I'm already very google-centric, using Google Calendar, Gmail, and numerous other free Google products for what I do. So having a phone which pulled it all together on a simple device makes me very happy.
I like the slide-out keyboard. It feels sturdy and not like cheap plastic like most other phones i've had.
The person complaining about the Trackball obviously has a defective phone. I've had the phone for months and haven't had the symptoms he's whining about. That guy is also comparing the Sony Xperia (a $900 phone) with the G1 at half the price.
The apps are very useful and nice. SnapTell and ShopSavvy actually save me money. I use Moxier and TouchDown to get my exchange emails. I use imeem, lastfm, and streamfurious to listen to music at work. After the latest upgrade, the Camera functions as well as any other camera phone with focus.
Google Voice search has been great to use on the road and when I can't look down at my phone to search for something nearby.
It already has Google Maps well integrated into the phone, but the last major thing I think it needs: turn-by-turn GPS is coming to the phone on the 24th of February.
I will be keeping this phone for a long time to come. Android updates will keep my phone going strong and up-to-date well into the future.