As an FYI to future readers...
I've tested all connections, Tmobile, Cingular, Verizon and Sprint, part of a companywide datacard rollout, and we settled on the Sprint ones. Their connection is quite good, better than Verizon's EVDO and a whole lot faster. And their compression software is intelligent enough to know when I'm on the VPN and when not. With other cards we needed to kill the compression due to major slowdowns as the card's compression software was trying to get access to our private network.
Also, Sprint gave us a good deal, free S620 cards and $45.00 a month unlimited data.