I've been with AT&T for about 7 years now. They've been great for me, no complaints.
Before I signed up with them, I tested Verizon for about two weeks. Got crappy coverage at home, and it wouldn't work at all in my lab. AT&T worked fine in both places, so they got the nod. Before that was a couple of painful years with T-mobile, preceded by PrimeCo PCS, which ended up becoming Verizon Wireless in most markets.
I now get pretty sizable discounts through my employer, as well as the ability to upgrade/change phones as often as I like, provided I extend my contract each time I do.
I typically go through about 3 to 4 phones a year, then flip them on Ebay when I am done with them. I have a good thing going with AT&T.