I have one, I've had quite a few smart phones dating back to the Handspring Visor phone module (It was an add on you put in your Visor (Palm pda) ). I've gone from Palm, to Windows mobile about 3 years back. I was pretty loyal to WM, but it was turning into a smoking turd. In an effort to make it better, they forgot what the device was all about, a phone. I was on a $30 for everything plan on Sprint and I couldn't take it anymore, the N1 baited me away.
It is an amazing device, but you can't talk about it and not compare it to an iPhone. My girlfriend has a 3gs. Some observations:
Screen - N1, way better
Build quality - N1 all the way, iPhone seams cheap compared.
Phone, 3gs wins hands down, especially speakerphone, the N1 is pretty bad
Battery life, 3gs all the way. N1 needs nightly charging.
OS - some parts of the N1 are less than perfect, A-, apple did their homework, A+, however, apple limits what you can do, there are no such constraints on the N1. Also, multi-tasking is not yet available on the iPhone.
You really have to use an N1 for a while to enjoy all the amazing subtleties that make is the great phone it is. Things like notifications scrolling across the top of the screen, not interrupting whatever you were doing. The google integrations that are there are amazing, however google docs is missing, and it hurts.
I will say this, if you aren't geeky, and just want something that works, get the iphone and don't look back. Its easy, well refined and well managed. If you like to tinker, the N1 is your phone.
