I've been really pleased with Tmobile overall, and their pricing structure just got a whole lot better, assuming you either bring your own phone or don't mind buying theirs on installment/buying unlocked on your own. The big caveat with Tmobile is that the service is awesome in urban areas, but so-so rural. There are lots of times at MW and Crandall where my data is slow (1g/2g) and dead spots on long drives. Since I don't leave the metromess much, it works fine for me.
The no-contract aspect is good, as is the unlimited data (they set speed limits after your allotted data package, but it doesn't charge extra or stop). Their phone prices are actually competitive, based on some recent reports on c.net or one of those other sites that compared the actual cost over 2 years compared to a subsidized contract phone.
I found ATT wireless to be as irritating as all their other services - crappy customer service, decent phone/data - but not stellar, constant billing errors and tough-to-follow pricing that seems to change as the various promotions fall off.
Everyone I know that has Verizon wireless loves it, says it always works great, coverage everywhere, fast data, and you pay for it. If you want the flat-out best cell service, they are it, but not really a bargain (unless you depend on it for business)
Lots of folks using the Walmart prepay stuff these days with success, but seems like its a little more effort and trade-offs to get the value.