OK, first we're gonna start with a bit of background here. I've had a cellphone since 2002 (I'm only 24 so...). Since then i've probably owned about 60 phones, at least 20-30 of those being smartphones. I've had Motorola's, Samsungs, Nokias, Palms, Sony/Ericcson's, Siemens, BlackBerries, the iPhone, other HTC phones and i'm probably missing some. As far as mobile OS's, i've used WM (2003, 5, 6, 6.1), PalmOS, iPhone OS, BlackBerry OS, Symbian and again, probably missing some here too.
That being said, the G1 is the best phone i've ever owned BY FAR. The ONLY complaints that i can't really call complaints are:
1. The battery life isn't grea, but that doesn't bother me. My phone is on a charger at work and at home so it's probably only off the charger for like 4hrs a day, if that.
2. I wish i could tether without rooting over something other than USB.
Those are my only 2 not big complaints with the phone. If the battery thing is a issue for you, you can buy a battery with double the life of the standard battery for $25.
I could go into how amazing the G1 is with ShopSavvy and Locale and all the other really, really cool apps, buf if you look around on the interweb a bit, you can see many other reviews of the really cool apps.
Honestly though...If my G1 failed tomorrow, i would go right out to a T-Mobile store and buy another one.