The simultaneous voice/data thing is an interesting phenomenon to me. Personally, I enjoy the functionality too much to consider switching from AT&T to Verizon or Sprint. Even though I would like to move back to Verizon, that one feature holds me to AT&T. But, having said that, I don't know one single person who is on Verizon with an iPhone (and I know many) who look at that feature and tell me that they miss it, or that they would consider switching to AT&T to have it.
It's just one of those personal preference things.