Two of the IBM's simply quit. No spinning when powering up, and the BIOS would sometimes see them, sometimes not. Probably a circuit board issue. One had a lot of important data that we sent to a recovery service (data was recovered for a hefty sum). One 120GB Deskstar was DOA.
The WD's always seem to have the same symptoms when they fail, or are about to fail, which is a loud pinging sound by the read/write arm banging around. We've had the other brands do this to, just much less often. I learned a trick when that happens that's worked a few times: put the drive in the freezer for an hour, immediately hook it up as a slave drive and get your important files off of it.
To be fair, the Western Digitals we use have been 20GB or less and used more intensively than the others.