Switched from VZW to T-Mo and couldn't be happier. I know this is about AT&T but I feel like T-Mo is really a threat to the two big boys right now and I'll explain why in a sort of backwards way.
Here's the thing: T-Mo's coverage is not in any way as complete as VZW or AT&T. End of transaction. I travel all over the PA/NY/Del/NJ area and found myself in tons of EDGE spots while on the highway. Speeds were passable and I could listen to Spotify with minimal breakups.
THAT SAID, Verizon also has tons of really goofy dead zones if you live out this way, and Verizon's dead zones tend to be TRUE dead zones. Just no coverage whatsoever. Either you have LTE or you're gone. I was driving out to New Jersey today and was on EDGE for a while but it's not uncommon to lose LTE and even 3G along that route.
Speedwise there's no difference in my limited experience between VZW and t-Mo. Standard LTE speeds in the northeast, meaning between 7 and 16mbps depending on where you are.
So, in terms of straight-up coverage and speed, Verizon > T-Mobile. Inarguable. So why did I switch and why am I happy about it?
Simple: their plans are just plain better. I might not have LTE in as many places, and I'm not gaining any speed whatsoever, but T-Mo gave me a hefty credit on my bill to offset my ETF from Verizon, and now I'm getting unlimited everything for $70 a month. Verizon was going to charge me $90 a month for 6GB of data and 450 minutes talk time, and THAT was supposedly a "deal" they'd give me because I was a long term customer. For a Spotify user, the ability to put on playlists and radio in my car again every day instead of having to download to my phone is WELL worth it.
In the end, I'm sacrificing some coverage for a much cheaper bill and unlimited data plus breaking away from the biannual upgrade (well, one biannual for another biannual, damn English language). Plus the other neat bonuses like JUMP or the little bit of free lifetime data on an iPad. T-Mo has coverage where I go, and the places it has less than perfect coverage are places I'm only hanging out for a little work, not to watch Netflix, so I'm not losing anything.
Is AT&T threatened by T-Mobile? Maybe not right now, but they will be very, very soon. So will Verizon. T-Mobile is gaining customers at a crazy rate, and with that means more towers, which means better coverage. The rep I talked to in Scranton mentioned that with the increases in customers they're looking at expanding the service.
People say it's unsustainable. My friends in England and Europe are BAFFLED at how much my cell phone bill used to be. T-Mobile is the future.