I did this for a while using a Verizon USB720 (USB dongle, Evdo Rev A). I live in the suburbs of Nashville, TN so service is a little spotty around my home. It takes a little effort to find just the right "sweet spot" to get a good signal to it. I ended up purchasing a Kyocera wireless router that was designed just for the purpose of sharing a mobile broadband card. The Kyocera (made by D-link, so there are D-link variants out there too) has slots to plug in your Express Card or USB modems. Once you set it up, you simply plug in your modem and power it up. A minute later, you have access to multiple computers. It is kind of buggy, but it's nice to have. It even came with a cigarette lighter plug so that it can be used in your vehicle. Plug it all in, toss it on the dash, and you've got internet access just about anywhere.
The service was expensive ($59.99 per month), and wasn't the fastest thing around, but it was the best offering that I could get in the sticks (until the local cable company get their act together). If I put the WAP in just the right place, I could get 900Kbits down / 256 Kbits up, and the latency is slow if you're a gamer, but totally reasonable if you're a more basic user (~80 ms).
When my cable company had an offer for both 5Mbit / 512Kbit cable AND basic TV for $19.99 per month for a year, needless to say I signed up for that.
Still got the mobile stuff though. Comes in handy! The mobile WAP was great when a storm knocked out power at work in the city. I even plugged our four Vonage lines in to it. Conversations were a little laggy, but we still were able to make and receive calls. It sure beat the hell out of having nothing and being dead in the water! :)
If you're a casual user that needs mobile access more than anything it's great. If you are heavy in to gaming or downloading music or videos, leave that to dsl or cable.