I've tried XBMC and Boxee on the AppleTV. YMMV, but as far as I've seen the AppleTV is just too slow to run either of them. They seriously underpowered that thing. It's rather depressing. It looks like they gave it about 2% more power than it would need to run iTunes-purchased HD movies and not an MHz more. To be fair, they don't have to cater to hackers, but it really is a slow box, and I think that underpowering it limited what they would be able to do with the platform in the future.

And even if I didn't hack it, it's a nightmare to try to convert video for the AppleTV (at least on Windows). Handbrake does a good job, but I still had half the videos I made fail on me.