I like the way AT&T does it mechanically. No fees to start or stop. Pay for a bucket of bandwidth and if you need more, buy more which starts that day. I guess there are no rollover megabytes, which would be a large improvement. At least you would have them if you needed them since you didn't use them.
Virgin resells the Sprint network which IIRC roams on Verizon as well. They only have 3G in LA, but coverage is wider than other offerings including mountain and desert communities. Their device is sold out online but retailers like RS, BB, WM have them. $40/mo unlimited, no contract.
Rocketman