Most MVNO's do not offer roaming, but a few do. I am currently using Republic Wireless, which is Sprint based, but includes all of the regular Sprint roaming on Verizon.
$20/month for unlimited Voice, Text, and Data! Really.
The downside is that they only have a few phones to pick from, and I will probably in a few months for a provider with an iPhone option.
Additionally, even if you like the cost structure of prepaid service better, you can't buy the same quality of service. The prepaid networks (and resellers like Cricket, etc.) are limited to a single carrier's network, and there are no roaming agreements for prepaid customers. So where a Verizon contract holder could hop on to a Sprint tower, a Verizon prepaid customer will have no service. Same with the other networks.