Virgin Mobile’s Broadband 2 Go service has long been appealing to me. They have a MiFi and the 3G plan is only $40 a month for unlimited data and more importantly has no contract. So you can sign up when you need to and not pay if you won’t be using it for a couple of months.
They recently announced plans to throttle bandwidth after 5 GB have been consumed. This isn’t optimal but it’s still better than overage charges or cutting of service.