I had Verizon. I hated it. Some of my complaints:
1.) Crappy customer service. I usually had to call twice (at minimum) to get anything taken care of.
2.) Sub-par phone selection. It wasn’t totally crappy, and I usually managed to find a good phone. Their LG models are good quality. However, the new and shiny phones were always late in coming.
3.) Dependence upon BREW. This was the main dealbreaker. Other phones, you could usually get some third-party apps to do what you wanted to do; not so with Verizon! You were stuck with their offerings, and their offerings were paltry.
4.) They liked to move my due date around, anywhere from the 16th to the 10th. It kept me on my toes.
The coverage, however, was great–I got reception almost everywhere. Sometimes big buildings were dicey, but that was about it.
I have AT&T now. Here’s their drawbacks.
5.) GSM buzz. Christ, but I hate GSM buzz. It proved useful once, when I found my on-silent phone by having someone call it and waving my speakers around, but other than that, it’s effing annoying.
6.) The coverage isn’t as good. It seems spottier and more finicky in my area. Ditto the quality of voice, though it’s possible that my hearing isn’t what it was.
7.) No LG phones, if I wanted one.
However, their customer service has been pretty good, and my due date is the 10th, period, end of story.
I have a Blackberry now, so some of these things don’t apply (namely #3 and #7, and maybe half of #2, since the Tour’s pretty nice). I’ve considered switching back to Verizon, but I probably won’t. This is mostly because my work goes through AT&T, so I get an 18% discount. Also, #3 really pisses me off on principle alone, even if RIM devices aren’t affected, 'cause it reduced my otherwise wonderful enV to a novelty phone.