I installed this on my MacMini and I am very impressed; it has almost the same functionality that my faithful XBMC (on the xBox classic) had, but it also supports HD content! (not really possible on the original version due to the limits of the xBox hardware).
1080p MKV files played without a hitch (maybe a little lip-sync, but that can vary in movies); 1080p WMV-HD played, but no audio (could be a bad encode) and was a little jerky at times. IMGs and VOBs for DVDs play with no problem. DivX, XviD, MPEG, MPEG2 and MOV all play. Please note that for the video playback I was running at 1920x1200- so it should run a little better at a native resolution.
The current beta version of XBMC for OS X will play slightly more content than my Vista Ultimate HTPC with my default Codec setup (ffdshow, Mastroka and AC3 filter).
The default setup utilizes the MacMini remote- but there are some features missing (like a 'Stop' button). I also see it has settings for 'Universal Remote' but I do not have my xBox remote handy to test. I was able to bang around on the keyboard and find a key that returned to the menu (tab).
The XBMC for OS X system was very stable- until I started installing scripts form the main menu. The Flickr script crashed on install and the Cranky Geeks podcast script crashes the application when it tries to download; I am assuming this is still a work in progress.
A little more testing and I may be looking at an additional MacMini for the roommate's system (so I don't have to burn DVDs for her).