Scott Beale of Laughing Squid has good things to say about the $50 (after rebate) Sierra Wireless Compass 597 EV-DO USB Modem on Sprint's $60 a month (not really unlimited) unlimited plan.
EV-DO has come a long way since I first started using it back in January 2006, we are now on Rev. A and the size and functionality of the EV-DO modems has improved quite a bit. I recently picked-up the tiny Sierra Wireless Compass 597 USB EV-DO modem using the Sprint network. It comes with GPS, has a slot for a microSD card and a port to connect an external antenna. Best of all, it is small enough to work on a MacBook Air without using an extension cable.
The software has evolved quite a bit as well, especially on the Mac side of things. The modem ships with Sierra Wireless' TRU-Install software, which automatically loads up when you insert it into the USB slot, then when launched installs the drivers and Sprint's SmartView software, all without the use of a CD. Activation was super easy and once complete getting online is a simple as hitting "connect".
