Since what I use with T-Mobile is an iPhone 4S, I don't think it would be valid for comparison to the iPhone 5, but I had been using T-Mobile 3G these past few months without any major problem until today when I was asked by the iPhone if I wanted to update the carrier settings. I then updated from T-Mobile 14.0 to T-Mobile 14.1 and powered off and back on.
What I know see is 4G instead of 3G and under Settings>General>Cellular the toggle has changed from Enable 3G to Enable 4G. I haven't yet used the 4G intensively enough because I am at home on WiFI but after trying it out for a bit, I do think it is faster than 3G was, so for now I am a happy T-Mobile customer. We shall see when I am out and about tomorrow morning.
I also now have Visual Voicemail which I did not have before.
It would be great if some iPhone 5 users could report their findings.