It could be worth a try.. I've never used a Wireless ISP (I just have cable, yay me) and I'm a novice at wireless, so I'm always wary of it, but it will probably be a bit slower than an average user's DSL, I think. Faster than dialup, like they claim, of course. How out in the country/in the city are you or how far away from the tower? It probably gets worse the farther out you are.
Still, security would be my biggest concern.. after reading up on their site and seeing that one graphical depiction it seems fine, they run on a licensed frequency range and all.. that's good, but if you can configure those modems too much, or if they don't do a good job of it, then you might well be able to turn one into a listener device to capture others' signals if they haven't secured theirs properly? Anyone know better? Mind you, that entirely depends on what the modems do besides capture signals from the Clearwire tower.. what kind of encryption protocol they use to communicate between modem and tower, which they don't say what it does or doesn't support on the website, I think.
And prepare to lose connection when there's a solar eclipse!
Maybe I'm wrong on it all though, but I'll be watching here closely to find out the answer too :)