Yup, there's a couple of threads on here detailing the experiences of others, but it seems like they work pretty well on desktop Macs. Personally, I've used CF cards w/ Acard adaptors (seen some on eBay in the ~$70 range) and cheapo IDE-CF adaptors in a few Quadras, a PCI Power Mac, a Color Classic, and a few IIcis that I gave away and have had no major compatibility issues. On 68k machines, stick with smaller cards to avoid glitches, and get a copy of FWB HDT or Silverlining to take care of drivers.
PowerBooks are another story. Some people have had their hands full trying to get a CF card to work on some machines. IIRC, people have had success with the 5300, and have struggled with the 5xx and the 150.
All you'd need for the PC (or any IDE Mac, for that matter) is one of the aforementioned IDE-CF adaptors. Which are dirt cheap. Buy a few different ones, because the build quality isn't always great. Anyone try a CF on a 630 or 580?