My 64GB 4S went back to Sprint yesterday. I don't know what I was thinking when I decided to leave AT&T. I'm grandfathered in with unlimited data, and a 23% discount as an IBM employee (which I never was - dude in the AT&T store hooked me up with it when I mentioned that I was distantly involved with IBM products, years ago). My Sprint experience was an unmitigated disaster. At home I'd get 1-3 bars switching between Sprint and "Extended" (Verizon roaming) towers, more often than not with a 1G data connection (the little circle symbol). Even when I got 3G, the download speeds were as horrendous as others are reporting. At the office, same story, though slightly worse - almost never a 3G signal in the building, and barely better outside with zero obstacles in any direction. This is smack in the middle of Tempe AZ, hardly in the boondocks.
I went back to AT&T yesterday and had my old 4 lit up again. They just reactivated my old account - discount still in place, unlimited data still in place, and $5 / 200 texts still in place. It's as if I never left. Ported my number back in minutes so no loss there either. I then went to the Sprint store and returned the 4S for a refund. Now I have to find a 4S someplace, will probably make an Apple reservation and pick one up at the store this week.
Moral of the story - if you're considering moving to Sprint, think twice. Maybe it's a better option in other cities, but here in Phoenix, they suck serious ass.