I've had tmobile, sprint, verizon, att, and alltell.
Let me tell you something, and that att is the best out of all of them.
Not only do I get very good service (network and customer wise) but it's just that it is much more easier to do and get done then any of the other companies....
First service I ever had was att and then I left it to all the other services and I felt like crap in every one of them (except att), I even got into a fight with 2 of the verizon people (they kept transfering me to someone in india..) and I had told them that I had att and every other service before and that they sucked and I will be paying the ETF just so I can get done with them.
They have bad service (and me being in a high industrial city, in LA) everywhere and there workers suck. I kept going to the store for verizon and kept leaving because they couldn't be anymore stupider when I tried to ask them a simple thing..Keep bashing att about there service but I don't care about bars and what not, my internet on EDGE is just fine and I have good call quality thank you very much, I don't need verizon and it's **** service. Tmobile i cant really say anymore since it's been a while but I just thought att was better IMO. In terms of customer service, network and ease, att wins for sure, and if your ever in the mountains, att is there for you.
One last thing, I am getting quite bored of my iPhone but only because the phone is so simple that you learn everything WAY too quick and that it is just too easy there isn't really anything left to do with it. I like other phones UI's where most things are confusing because it has more curiousity to it.