I have a T-Mobile phone (HTC Sensation) and I love wifi calling. It works beautifully where my cell coverage is bad and there's a good internet connection (home, work, a ski lodge I was at recently).

I'm on T-Mobile more or less accidentally, and before I got my smartphone, had absolutely no loyalty to them. I didn't even realize the phone had the feature until I got home and started playing with it -- they didn't play it up at all in the store. It would be hard to pull me away from T-Mobile. A competitor would have to offer a huge price break, or wifi calling.

Really puzzling that they don't make a bigger deal of this feature.