Nice write-up on Plex vs XBMC for a homebrewed multimedia setup. My 2 cents, I have used XBMC in the past, it has more power than Plex, but I agree with the author here in that it can be a pain in the ass to setup.
http://lifehacker.com/5991757/should-i-use-plex-or-xbmc-for-my-home-theater-pc