My route to the N1 is as follows:
Nokia E70 (AT&T) > iPhone 2G (AT&T) > Nexus One (T-Mobile)
I think the biggest factor for my switch was cost savings. Having the iPhone 2G, I was paying $98/mo for 450 minutes of voice + unlimited text and 2G data. On T-Mobile, I only pay $60/mo for 500 minutes of voice + unlimited text, mms, and 3G data.
I live in Philadelphia, where T-Mobile 3G coverage, so far, has been really impressive. In my home and my neighborhood (Fishtown), I consistently get about 300k - 700k download speeds, but at the house, I have WiFi, so no big deal. My girlfriend still has an iPhone 3G and can only get 500k at best, no matter where we go. We just went to the Port Richmond Target, and I clocked 2.5 mbps while she still only got 500k on the iPhone.
Network-wise, I'm more than pleased. This may not be the case if I travel outside a major metro, but I don't that often, so this works great.
A few things I like better on the iPhone:
The Facebook app is way better. I have to use 2 apps on the N1 to get the combo of functionality that I was used to in the iPhone Facebook app.
The way that you select text and move the cursor around on the iPhone is much better. Maybe there's an easy way on the N1, but I haven't figured it out yet. I can never seem to select the text I want or move the cursor to the spot that I want. Not a dealbreaker, but annoying sometimes.
The OLED screen on the N1 absolutely eats battery life. Admittedly, I am fiddling with the phone a lot right now since it's new, but I only get about 24 hours of use out of the thing before it has to go back on the charger. This is also compared to the iPhone 2G, which was good for about 2 full days of use if I wasn't on the phone a lot. I never had a 3G, but I hear they can be battery hogs.
Apps that I miss:
TweetDeck and New York Times. I also played this game a lot called Scramble by Zynga, that's not available for Android (yet).
So, overall, I'm extremely happy with the N1 and I only miss the iPhone sometimes. The $40/mo in savings + blistering 3G network speed sure helps to ease the transition.