I travel quite a bit and mostly to major metro areas so I should have good 3G coverage from AT&T. I've used several AT&T 3G modems, my iPhone and most recently a Verizon MiFi. I typically get a better connection with Verizon than I get with AT&T.
Of course, they both treat their customers like crap. My wife and kids are on T-mobile and while they have worse coverage, they at least know how to treat customers.