I made the switch from Verizon. I love Verizon's coverage, customer support and reliability. But the 8320's UMA (calling over Wi-Fi) put my mind at ease about the possibility of not having T-Mobile signal at work, home, or when staying with friends and family. So far I do have cellular signal in those places, though.
I was out of contract but you do realize that they pro-rate contract termination fees, right? If you're near the end of your contract you'll pay peanuts to get out of it.
T-Mobile service has been great so far but I do realize it's always going to be more limited than Verizon, especially in rural areas. I do not miss the high-speed EV-DO that I once had because it was $45 and the devices Verizon offers are lethargic and terrible on battery life, rendering the internet access almost useless to me because I couldn't use it for even a few minutes without killing the battery.
My opinion on "waiting" is that you should never wait for Verizon. Look at the people who waited for the LG VX-8100. It was over a year and they crippled the life out of it. They put their rotten standard UI on it like they do all feature phones. Look at the people STILL waiting for the XV-6800, which was supposed to replace the XV-6700 *last October* and still is not out. Also consider that Verizon is never hurt by "rumors" of new devices because it keeps people on board.
Get out while you can and don't look back. Verizon will have better devices in a couple of years and then you can switch back.