Why Plan 9 indeed. Isn't Plan 9 just another Unix clone? Who cares?
Plan 9 presents a consistent and easy to use interface. Once you've settled in, there are very few surprises here. After I switched to Linux from Windows 3.1, I noticed all manner of inconsistent behavior in Windows 3.1 that Linux did not have. Switching to Plan 9 from Linux highlighted just as much in Linux.
One reason Plan 9 can do this is that the Plan 9 group has had the luxury of having an entire system, so problems can be fixed and features added where they belong, rather than where they can be. For example, there is no tty driver in the kernel. The window system handles the nuances of terminal input.
If Plan 9 was just a really clean Unix clone, it might be worth using, or it might not. The neat things start happening with user-level file servers and per-process namespace. In Unix, /dev/tty refers to the current window's output device, and means different things to different processes. This is a special hack enabled by the kernel for a single file. Plan 9 provides full-blown per-process namespaces. In Plan 9 /dev/cons also refers to the current window's output device, and means different things to different processes, but the window system (or telnet daemon, or ssh daemon, or whatever) arranges this, and does the same for /dev/mouse, /dev/text (the contents of the current window), etc.
Since pieces of file tree can be provided by user-level servers, the kernel need not know about things like DOS's FAT file system or GNU/Linux's EXT2 file system or NFS, etc. Instead, user-level servers provide this functionality when desired. In Plan 9, even FTP is provided as a file server: you run ftpfs and the files on the server appear in /n/ftp.
We need not stop at physical file systems, though. Other file servers synthesize files that represent other resources. For example, upas/fs presents your mail box as a file tree at /mail/fs/mbox. This models the recursive structure of MIME messages especially well.
As another example, cdfs presents an audio or data CD as a file system, one file per track. If it's a writable CD, copying new files into the /mnt/cd/wa or /mnt/cd/wd directories does create new audio or data tracks. Want to fixate the CD as audio or data? Remove one of the directories.
Plan 9 fits well with a networked environment, files and directory trees can be imported from other machines, and all resources are files or directory trees, it's easy to share resources. Want to use a different machine's sound card? Import its /dev/audio. Want to debug processes that run on another machine? Import its /proc. Want to use a network interface on another machine? Import its /net. And so on.