Roku are awesome too. We've got a bunch of them here. My kids have them in their rooms and with Netflix, they can't be beat.
I'm thinking of dumping everything and building a Hackintosh desktop to as a central server with Apple TVs hooked up to them.
Plex is based on XBMC, it's the idiot proof version. You can't customize it to the level you can XBMC.
OpenELEC is a really nice build based on XBMC that adds a few features and already set up customizations out of the box:
http://openelec.tv/get-openelec
XBMC has been around almost 10 years now I think. Nothing really compares.