I'm in the exact same boat as you OP. I bought a GSM GNEX the day it was available (and paid over $650 for it) and have been happily using it ever since. I like it, don't get me wrong, but I also picked up a slightly busted screen Focus locally and have been toying with WP7. So far, I have put my SIM in the FOCUS for a few days here and there to "warm up" to WP7 and have been happy. I used to use Nokias in the past and always liked their Drive app, as I have not owned a standalone GPS for many years.
I got the itch to get a Lumia over the weekend and snagged a black one on eBay for $280. Pretty good price and I don't have to burn my upgrade on one. Even free would make me worry as we *still* don't know about the future of the Lumia's OS/updates. /sidebar
The biggest thing I am concerned with about the platform switch is my deep ties to the Google ecosphere. I use Google for literally everything and the apps I use daily aren't on WP7 (2Do, Alarm Clock Extreme, Chase Banking, Goole Voice, Google video talk, Google +, various other apps, etc). Yes, I can limp by on WP7 with substitute apps, but having them natively on Android has become second nature. Also, Google navigation, although not totally offline, is superb. Nothing can beat saying "Navigate to 1100 E, 435 S" and off you go. Drive may be able to do this, but having to type it in is a nuisance. Also, transit directions and live updates are just better on Google Maps. I use the MAX in Portland a few times a year, so I'm not very familiar with it. Nothing beats using Maps to track my progress and stops in real-time. It has saved me some serous frustration. Nokia Transit doesn't support Portland (yet?). Also, Whatsapp, while on WP7, is pretty bad compared to the Android version. Slow to open and laggy when loading messages. It's sometimes a chore to use (and I use it many times a day). Lastly, I have two original batteries for my GNEX and usually don't ever need the second one. It lasts a full day easily (stock, rooted) but it's nice to know I have a spare in my pocket. Yes, a seperate battery pack would work, but it's far less elegant to have something protruding from the top of the phone.
I think my curse was I have the GNEX with stock Android 4.0.4. I (and most everyone) would find "dumping" Android much easier if I had a crappy Android phone with manufacturer OS skins and still running 2.3 or something. ICS really is very smooth and doesn't suffer many of the issues that people ***** about with Android. It's silky smooth in every respect.
All of that aside, I really want to like WP7 and hope the Lumia works out. In case it doesn't, I have a microSIM adapter ready if I give up. I can put my SIM back into my GNEX and eventually sell the Lumia if the "beta test" fails. I'm 50/50 now. Time will tell.