I haven't used the card, but I bought a USB cable to attach my phone (a Sanyo 5300) to my laptop for the same purpose. It works, but the reliability is really random. When it's working well, the speed is great -- I've gotten as high as 200 kbps on good-sized downloads.
However, they force all traffic through a proxy server, and it can get slowwwww. They also compress the f*ck out of graphic images, etc. Connection reliability is pretty random. I can get a good, working connection for 2 hours or more, or at times I have trouble keeping connected for more than 10 minutes.
My usage has been primarily from several locations in urban locations in Chicago and Minneapolis. I have not tried using the service from a moving vehicle, so I can't speak to that (although I doubt it would help).
Also, be aware that they do block ports. I could use HTTP and HTTPS without problem, but IMAP and SSH were blocked. I don't use POP3, but I imagine that probably works.
For the extra $10/mo on my bill, is does come in handy on occasion. For the more expensive PC-card plan, I would hope it would be more reliable than I've found it to be.
--Dan.