[Primenall]

Have you set it to roam only and do the calling test, like ImCoKeMaN said it will probably be roaming on Verizon, at least you would see Verizons coverage before you switch.

You wouldn't want to do this more than half your minutes with Sprint or they may terminate you for using too many roaming minutes(of course it sounds like you may want that anyways).
Plus Data roaming off Verizon is only 1X for Sprint phones.

As for jumping ship, like was said going to Verizon won't solve your problems, if you have to leave I'd get the Kaiser it's awesome, if only Att wasn't slower than a snail around here I'd probably have one myself.

Don't get me wrong, it seems to have some bugs itself from what I've read but it also has a few great improvements too, More Ram-- GPS--- 3.0MP Camera.