it's not just plug N play. I bought an expressCard(from syba) for e-sata and could not find the necessary drivers to make it work with my mac. After hours of searching online for help(their website was useless), i took the card back and got a much more expensive express card from meritline
the meritline merax expresscard/e-sata had a "supports mac" logo infront of the box. after installing the drivers, it has worked flawlessly on my macbook pro. drivers not only came with the card on a cd but they were easily available online for download. (website much more useful)
I would have gone for a more widely known brand like belkin or SIIG, but fry's didn't have their cards instock at the moment. I had to settle for the meritline but I am glad I did because I've been very happy with it.