I have just Installed XMBC on my Ubuntu system this morning and it works without a hitch, I have used Microsofts media center before but the Open source version tops it.
It is real simple to install with Ubuntu and there were only a few local setting to adjust for the weather and file locations. You can get it HERE
Be sure and read through this and install all you repositories,
If you are up for a big change you can download the new version of Sabayon if you look through the downloads and find the ones ending in MCE they have XBMC included., Here is a screenshot of Sabayon in action.