My connectivity and service have been okay, I guess. My reception on T-Mobile was a bit better (and it's not just because of the phone; if I put my old W580i on AT&T my reception is definitely worse than it was on T-Mobile). But that's not the main problem.
I never once got a "promotional email" or "free promotional SMS" from T-Mobile in my 7 years with them. In the 6 months I've been with AT&T I've gotten countless SMS and email spams from them. I have unsubscribed many times. I still receive them. They are disruptive and annoying, and so far the only one I've even vaguely cared about is the information about getting free AT&T WiFi hotspot access through my iPhone; everything else has been crap like "get American Idol ringtones!" and "upgrade your phone!" and other such nonsense.
I guess they aren't content with already getting close to $71/month from me for what little I use the iPhone for (I use maybe 200 minutes and 10MB of data a month, which was costing me about $40/mo on T-Mobile).
I think it's pretty telling that one of AT&T's "features" is now a "flexible ETF program," so now it's a feature that I'd have to "only" pay $145 to get out of this one-sided contract. What crap.