I've been sailing (and living aboard) with a Sierra Wireless 860 card on AT&T for 3 years. The card works very well. I've had 3G throughout the upper Bay and EDGE or 3G from Deale south to Norfolk.
The quad-band coverage has allowed me to hook-up (at varying and generally high cost) in the Bahamas, the BVI, USVI, England, Northern Europe, and Scandanavia.
I dropped the card a few days ago and it died. I found another one on E-Bay and am waiting eagerly for it. My understanding (no personal knowledge) is that the newer cards (Sierra Wireless 881 and various USB dongles) don't perform quite as well. YMMV.
I'm planning to stick with the 860 until the Novatel MiFi is out for GSM.
Verizon seems to have the market locked for the RV crowd. High-speed coverage is definitely better throughout the mid-US. AT&T is as good or better in the major sailing centers of the US East Coast, and has the advantage (with the right hardware) of international compatibility. I really like pulling into port or getting off a plane to have phone and Internet working without grief.