I'm pretty sure Verizon's MiFi is contract only (and $60 for 5GB data). Virgin's just recently became unlimited, but I /think/ it's always been contract-free, it just used to be 5GB.

Virgin's got a public awareness problem (at least among the people I know), however, so for most people I bet the only known options were from Sprint, Verizon, AT&T, etc.

IIRC you could buy a no-contract MiFi but it was much more expensive.