Leopard only
From the faq:
"Tiger support is not a near term goal, but it is a possibility in the future. It's obviously quite possible to get XBMC for OS X to work on Tiger, but the team has determined that it is also non-trivial. As we get closer to a 1.0 release we will poll the community to see how much demand there is for Tiger support."

I have tried VLC, Mplayer OSX, Perian and Niceplayer (with Perian) for watching *.mkv files. All of them are fine players and have their merits, when watching mkv files up to 720p. However, being on a Macbook Pro I need the players to support both CPUs for 1080p playback otherwise there is frozen frames (VLC), choopy playback (Perian) or video slowdown leading to lip-sync (Mplayer OSX).
XBMC gives me hope that there will eventually be a method for watching those fine HD mkvs on a mac (something the windows users are already doing). It played all the problematic files fluidly.
The only trouble is that I have to restart the program in order to start a different movie file since it crashes a lot. But the playback power is great since it uses >100% CPU (checked via Activity Monitor) for 1080p mkv playback.

By far the best media app out there for OS X. Once Apple Remote integration comes to fruition I can't see why you'd use anything else.

I have been using this since 0.1.1 or 0.1.2, I don't remember. Excelent work you are doing here.

v0.4.5 has been released n 08APR08!

The Apple Remote work is taking a bit longer than I expected, so another release is probably a good idea, to share some of the bug-fixes and new features that are complete. Here are the changes in this release:

* NEW: High quality software upscaling is now supported. You can specify preferences in Video -> Player as to when the scaling is enabled and what scaling algorithm is used. This makes a huge difference when viewing SD content. It takes quite a bit of processing power, but the good news is that decoding SD content doesn’t.
* NEW: Monitor blanking is now supported. When selecting a full-screen mode, you have the option of telling XBMC to blank the other monitors. The preference is in Appearance -> Screen. I suspect Martin will be happy that he can stop throwing a sweater over his iMac, which sounded like a total fire hazard to me.
* NEW: The built-in web server now works. Among other things, it serves as an alternate remote control for those of you using a laptop as an expensive remote. Make sure to set a port above 1024.
* NEW: You can now store your own skins in the Library/Application Support/XBMC/skin directory, to avoid having to move aside and reinstall skins every time you upgrade. Thanks to d4rk for coding this up!
* FIX: A crash when reading WMA files.
* FIX: The OS X mouse cursor sometimes made an unwelcome appearance in full-screen mode.
* FIX: Settings that made no sense for OS X have been removed to reduce confusion.

The XBMC source has been updated to near-trunk as well, which means a host of improvements and fixes. Note that the new music library features may still have some kinks, but hopefully things work pretty well for you.