I just switched over to T-Mobile from ATT … well actually a couple months ago. Here are some thoughts from my side:
For this, I unlocked a Lumia 900 and an iPhone 3GS.
Speed: There aren’t very many LTE towers that work with ATT phones. I found some in Bellevue and Tacoma. Haven’t found any in Seattle proper yet, but I’m told they exist. EDGE speeds…. are abyssmally slow. Remember dialup? Yea. That. 4G is decent, but the LTE is where it’s at. So far I have only gotten about 4MB down and less than a MB up. So… it’s decent.
Coverage: T-Mobile did a bunch of upgrades to their network. When I first tested the port last year, the tower switching was terrible. I occasionally had to turn the phone off and on again to get coverage. That has been fixed.
SMS / MMS: I’ve noticed that some text messages or MMS messages weren’t getting developed and that was because the APN settings weren’t correct. Changing those. Worked.
Tethering: I can do that now.
Price: I have the 30/USD month plan… 100 minutes of voice with unlimited data. I’m saving about 90USD a month. So … there’s that.
Overall…. pretty happy with the amount I’m paying for the service I’m receiving.