ive been researching this myself, here are my notes. I live in the desert side of WA.
in order of best deal with speed as priority and price secondary:
clearwire wimax very fast 4g, no coverage in my area, no voice (usb 4g modem)
Platinumtel 60bux sprint 3g unlim data, unlim voice (sprint 3g network)(requires tethering "hack" from phone)
millenicom 69bux unlimited data, no contact,, no voice (sprint 3g network)(free USB 3g rev-a modem)
sprint 3g/4g 5gb/unlim 69bux good 3g, poor 4g cov (may require tethering "hack" from phone)
sprint 3g 5gb 60bux good 3g (may require tethering "hack" from phone)
vzw 5gb limit 59bux good 3g coverage (usb modem data plan)
att 5gb limit 60bux good 3g coverage (usb modem data plan)
att iphone "unlimited data that isnt really unlimited" 70bux good 3g cov, 450min voice (requires tethering "hack" from phone, att frowns)
tmobile very poor 3g coverage, but cheap (requires tethering "hack" from phone)