I'm personally not all that familiar with it either, but I have read that it's a much cleaner and more modern equivalent to Unix, applying the "everything is a file" paradigm to far more. In Unix, a lot of behavior got tied into ioctl and binary sockets as the need for additional features grew. Plan 9 is far more file-oriented and textual in its interfaces, and relies more on file servers (i.e. eschewing an ftp command for an ftpfs filesystem). ESR believes that it may have died just because Unix was "good enough", which sounds reasonably plausible to me.

If someone else knows more, I would also be very interested.