I've had 3 of the last 4 Windows Mobile offerings, I didn't jump at the first iPhone, but I did try out the 3g when it first shipped. So my wife was not surprised when I left the house early this AM to stand in line for one of the first G1 phones in Seattle.
It's not as slim and sleek as the 3g, but what it lacks in footprint it makes up for in sheer geek-ability.
I've had a hard time doing anything today other than fondling this lovely piece of gear and downloading (for free, for now) every app that I would ever want to think about using.
If you are a Facebook user, you'll find the process of adding apps to the phone familiar. It lets you know what services you are exposing to the app.
I've read several reviews, and yes -- there is room for improvement but... OMG.
I spent a long weekend trying to jailbreak my G3 just so I could capture videos and secure (crypto) email. With the G1, it's Prego baby -- it's all in there.