Ok, so HD sharing on the PC side with the Airport Extreme seriously sucks. The question is, how well does it work on the Mac? I'm happy to report that it works much better on the Mac side (not surprisingly). While it still only maintains a 3-4MB/sec transfer speed when copying files to the shared HD, at least it works. Unlike Windows, it doesn't seem to die in mid-copy like it does on the Windows side. It works reliably. This is a good sign. Because of that slow transfer speed, it's not practical to run programs full time off the shared HD, but it is still practical to use as a backup HD.