I have been trying T-Mobile for the past few weeks. I used a prepaid SIM in my Galaxy Nexus at first, and determined that the service was good enough to port my VZW number over to the Value plan (so I can get a higher data cap before throttling).
There are some trouble spots, but that applies with all carriers. Over all, the service seems to be pretty good. Some buildings will fall back to EDGE, but that's no big deal (VZW will fall back to slow EVDO or even 1xRTT in many of the same buildings).
Outside the bigger cities, there's definitely a lot of 2G coverage. AT&T does have HSPA+ in most of those areas. Check the map at
T-Mobile Personal Coverage Check to make sure there are no holes in coverage where you need it. Signal strength is hard to estimate so if it looks like everywhere you go is covered, the best thing to do is to try the service
For rural coverage, there is some evidence that T-Mobile is starting to make some improvements. Nothing new in NC yet, but of course it takes time.
I only called customer service to port my number. Unfortunately it was outsourced (I believe the entire porting department is overseas), but I could actually understand the rep and the process was very smooth. I'm not one to call customer service much anyways since better answers can usually be found in forums (this applies to all carriers).
My primary reason for switching is to get away from all of the carrier BS on handsets and stick with Nexus devices going forward. Verizon's poor handling of their Galaxy Nexus updates annoyed me, and the locked bootloader on the Galaxy S3 was the last straw. When the Galaxy S3 was announced, I told myself that I would see how Verizon handled updates vs. the other carriers, and that would determine whether or not I would stick with them. They managed to screw it up on day one.