In my opinion, XBMC is what makes owning the xbox worthwhile. I too am very tempted to change from UnleashX as my main dash to XBMC. The only reason I haven't is because at one time XBMC wouldn't rename file names when ftping, but I think it does now. So when I get around to it I'm going to make it my main dash too. There doesnt seem to be very many XBMC themes around. Don't get me wrong, I love the latest Project Mayhem theme, but its nice to change things up every now and then.
The only things I believe that can be improved on is:
-DVD playback function (Needs more options. Try playing a tv show boxset, its much easier to use DVD-X2 dongle free DVD player, because it acts like a DVD player)
-Hmm that's all I can think of at the moment, but if possible in the music options if they let you actually adjust the fade-in and fade-out for the cross fading, I would cry. =)
Thanks to the people responsible for the XBMC, greatest entertainment program ever!!!
~Jason