I'm not sure about him, but my mifi is unlimited as its part of a large business account. We have confirmed and have in writing from sprint that this is the case as it is with all our data only plans (we have quite a few as part of a yard management system).
Some of the biggest advantages are being able to move the mifi for better signal. There have been numerous hotels with crap internet that could only get a decent signal with the mifi in the window (or even on the balcony).
Mifi charges off the same cord as my blackberry and plantronics headset so I always have a cable handy.
Mifi allows my pc and the wifes pc and my daughters touch and the ipad, ect to connect when at places that dont have wifi (which outside of metro detroit is just about everywhere).
Biggest advantage for me is i dont pay for the mifi. But if I did, i would still probably go the mifi route. I have more then enough content to not need wireless all the time, and the 4 hour battery of the mifi is long enough to get me between charges.