I've been using an Alltel EVDO data card for 4 months shy of my contractual 2 years, and it has worked well for me. The contract was "unlimited data" and when Verizon bought Alltel, we were assured that all terms of the original contract would remain in effect until contract expiration.
Over the weekend I noticed I suddenly could not get a 3G connection, only a 2.5G connection, and the speed was rather slow, so I stopped at the nearest Verizon store to inquire what that was about.
They had changed the service somehow, and I was told that to get back into 3G-land, I'd need to upgrade. After some negotiation back and forth, I left with a brand new USB modem-card (cost, $52.50, on which there is a $50 mail-in rebate) and a new contract for 2 years with the old Alltel unlimited bandwidth for the entire 2 years.
That's the good news. The bad news is that I had not had to install any Alltel software on my Mac to get the old card to work. The new card required some software, put up a fight to get properly installed and running, and which appears to be buggy enough to lock up the computer, which is really hard to do on a Mac.
What I've found is that the Verizon software was essential to get the card configured, even though it took a fair amount of fighting to get it to do what was needed. But the software appears to be otherwise un-necessary. I've been running since then, off and on the net, without it. So if you can get past the initial fight, never bother to run the Verizon stuff again and you'll be fine.