I just installed the SVN version of XBMC, so that I could use my newly installed CrystalHD card (which it supports). I was quickly disappointed to find that deinterlacing is broken in the Linux version of XBMC! Without deinterlacing, you will find it very difficult to play DVD movies or recorded HDTV.
This truely SUCKS!
I don't know what the other posters find so good about XBMC, but I can't even watch my MythTV recordings with it, so it is useless to me. The deinterlacing problem was first reported back in Sept 2008, so it must not be very important to them. Looking pretty is important to them, and they spend a lot of development on that.
MythTV, and it's GUI gets a lot of knocks for not being very pretty. But it can deinterlace most mpeg files, and that is what is important!