I haven't used Verizon, but I have used ATT and currently have a G1 with T-Mobile. Data speed has always been better with ATT, but the rest leaves a lot to be desired in my opinion. That said, the speed on T-Mobile isn't as bad as I thought it would be. On my old phone data speed was atrocious, but it's pretty usable on the G1. What's interesting is that I live in an area that T-Mobile claims does not have 3G, but in many of the places I go the phone says I'm on 3G.

It's probably worth noting that T-Mobile costs quite a bit less than Verizon and ATT. They also offer some neat features like WiFi calling (not on the G1) that the others don't.

In any case, it really sucks that people who don't like T-Mobile have to wait to get their hands on Android. I guess the bright side is that it will eventually be on other carriers unlike the iPhone. I probably would have gotten one of those long before Android was available had it not been an ATT exclusive. These carrier exclusives are terrible for consumers, but the situation only seems to be getting worse.