FWIW; I have the Virgin MiFi here, and it works quite well. Speeds vary a LOT. I get around 600 Kbps at home, but have seen just under 2 mbps at other places. Sprint has good coverage. Topups are easy too. Seems like the perfect gadget for emergency online access. Walmart carries the Virgin MiFi for $119, but is currently out of stock.
FWIW; you can add 3G access to your Comcast cable account for around $20/month, plus $125 for the MiFi.