A friend of mine who's got an iPhone 4S got it unlocked and switched from AT&T to ST/AT&T, and he said his data speeds jumped up considerably from what he was used to.
I've also heard from many (pretty much on AC) that they've had issues with ST. Right now I'm in no hurry to jump ship; I'm just keeping tabs on things so I know the history and have a sense of where everyone is moving (industry-wise) when I do eventually re-evaluate staying with T-Mo or moving.
For me, one factor is cellular coverage. Where I live, it's great. Where I normally go is fine. Where I work is also fine, so long as I don't go inside the building at my job. FGCU is mediocre all over campus. Sooooooooo... not sure what I want to do.