I had a T-Mobile PCMCIA card for a year from Jan 2007, as I lived somewhere that had good coverage but no BT line for a few months. The access was through a proxy that compressed jpgs massively unless you reloaded the page and they were a bit funny about VOIP.
Latency was crap, don't think it did VPN or anything clever but the sustained download speed wasn't bad at the time - 1.7meg IIRC and it was much better and less costly than using 3g data on my O2 phone tarriff.
My experience is prolly a bit out of date.