I was one of that 40%. I switched to Sprint because AT&T didn't offer an upgrade path for existing iPhone 4 customers to the 4S.
My Sprint download speeds in Downtown Seattle, not exactly Bumblenuts, Nebraska, were atrocious. If I was lucky, I could get over 100kbps down (yes that's hundred not thousand).
I recently switched back to AT&T with a used 4S from eBay. I lost my unlimited data grandfather status, but I wanted tethering anyway. And other than tethering, I rarely use even 1GB per month.
I did find out that if you complain loudly enough, file a BBB complaint, and make them spend hundreds, if not thousands, of dollars researching why their network sucks, Sprint will let you out of the contract without paying $350.