I've got one that I use with my PowerBook. The sound quality is pretty good, and the card is usable. But the driver (I'm using ASIO under OS 9) is inefficient, as I find that CPU usage maxes out sooner than when I use a soundcard that relies on the Propagamma USB-ASIO driver. I think this means that the driver is pretty poor, because the PCMCIA bus should be much faster than the USB bus.
I think in principle a PC-cardbus adapter does not have to be inferior to the Firewire/USB 2.0 boxes. The RME Multiface, for example, has an excellent reputation. However, the VxPocket is a lot cheaper. Now it's down to under $200, a price that should make it tempting for more people.