Really awesome, but 1900MHz doesn't penetrate the low-e coating that my employer uses on its office buildings. AT&T's 850MHz HSPA and 700MHz LTE do, so I can't switch, bad though I may want it.
If you get ANY signal through, you can use a signal booster, like CelFi: http://cel-fi.com/ . All they need is electricity and a window (to allow decent GPS signal).
They're 500$ new, 200-400$ on eBay, and free with T-Mobile subscription of 18mo (and dealing with some phone bureaucracy. This may seem like a lot, but it's a lot cheaper than paying 750$/yr more for data with AT&T, plus it will benefit your whole office.
I believe that T-Mobile offers WiFi calling to help combat the issue. According to the presentation, T-Mobile is hoping to improve in-building coverage by 15-20% through this network modernization (including distributed antenna systems). I'm not saying that should cause you to reconsider, but it's possible that T-Mobile will be improving in that area as well.
