Working and bug free are two different issues (Works on iPhone4, though its currently frustrating to use. taken from their website in their words). From what I remember, XBMC was so full of bugs on Leopard, I would not call that "working" as it was quite unstable .... Go figure the Elan and the OSX team decided to work on future support not past operating systems, 10.4 and 10.5....
Go figure? Plex decided they didn't want to bother to support existing users and so they decided to play the "go buy a new machine" game. That's their business, of course, but it means that it will not run on 5 out of my 7 machines here (2 Apple TV Gen1, 1 PowerPC G4 Mac acting as a media server, 1 2008 era Macbook Pro, 1 Hackintosh Netbook and a PC running XP plus an ATV2 still in the box unopened). For example, Plex doesn't work on Tiger and therefore Plex doesn't work on the first generation Apple TV and never will. Plex doesn't work on PowerPC and never will. Plex "sort of" works on Apple TV2 if you call "working" requiring a very fast host computer to transcode video for a device that is fully capable of doing itself. If I have to buy a fast computer to transcode (i.e. I cannot use that computer for other tasks while it's wasting CPU time to transcode), I might as well just buy a MacMini or home theater PC and be done with it. It's almost pointless in that sense.
XBMC works with PowerPC in both Tiger and Leopard. XBMC works with Apple TV 1 & 2 natively (no transcoding). XBMC also works on other platforms (x86, etc.). In short, XBMC works on 7 out of 7 machines here.
My only complaints about XBMC have been the lack of chapter support for M4V (added in 10.0) and the inability to read M4V meta tags (supposedly coming in a future release since it seems that their tag reader for MP4 music files can read the tags on M4V as well).
Boxee would be more useful on Apple TV 2 since a lot of the content does not play well on Apple TV 1.
Having to use a Mac to reboot an Apple TV 2 is not very appealing, though. I was planning on replacing one of my Apple TV generation 1 units with a 2 (and moving the old one to a bedroom or something) since the Netflix app is nice and now that XBMC is here, it seems idea. But I think I'll wait until the frame drop/bugs/kinks are worked out. Not having to convert non-M4V to M4V is the most appealing aspect of XBMC. It's a shame Apple refuses to support other standards like AVI and MKV in iTunes. If they did, it would all be unnecessary, IMO.