The coverage is improving, but I think ATT/verizon still have better coverage right now. regarding VOIP: I've used skype a number of times, and it works adequately. It is not perfect, not as good as a regular cell phone call (there usually is a little lag.) but is acceptable. Gizmo does not work well over 3g for me. The voice call is not smooth, it is always choppy.
BTW I use an N800 tetherd to an SE TM506 with unlimited data from Tmo for $19.95, and I'm on a 4 line family voice plan with the rest of my family. We share 1000 minutes(we never go over, since tmobile calls are free, and we mostly make those), and pay approximately $130 or $135/mo for the bundle I think...