I moved from AT&T (been a customer since the first iPhone) to Verizon back in February and left my wife with AT&T so that I could get the new phone when it came out. I have loved the move. I consistently get more and better coverage on Verizon. And I have paid pretty close attention too as my whole family is still on AT&T with iPhones. While there have been a couple times (as with any carrier) where the speeds/coverage wasn't what I would have liked, the overwhelming majority of the time things are awesome. The downloads may not technically have as high a rate, but they seem to respond more quickly (initial startup, ping, etc.), perform more consistently once the connection is established, and in many real world situations out performs my families AT&T iPhones on speed (with full bars on each, and despite the theoretical max download speeds).
I will hopefully be done with AT&T as soon as the port of my wifes number completes. I have been waiting over 5 hours for it to happen.