The monthly rate on mifi is TERRIBLE... furthermore, they don't even offer unlimited!
The MIFI, while a nice to have is VERY Expensive to the ADHOC Data plan offered by ATT. The limit of 5GB on all the carriers in the USA is lame, and the unlimited that is being offered by ATT for the iPAD is an amazing deal for the iPAD.
If you don't plan on having 2-3 other devices that you carry with you and also carry the MIFI with you (totaling at least 3 devices counting the MIFI). It would be hard to justify the $50 for the MIFI and $60+TAX!! a month (under a 2 year required contract) for the MIFI.
I would avoid the MIFI, and tell Verizon that when they can offer a month to month service, even with a lower cost .. NO SALE..
That being SAID:
You can use Virgin Mobile USB modem ($99) that is on the Sprint Network for $60 a month (5GB) with no contract This is a disadvantage is you would need to use some sort of internet sharing on a MAC/PC to give yourself access to the internet.