So, for a very long time, I had an XBMC box setup on my projector, got a AppleTV, pretty quickly stopped using the XBMC machine, and now I've since repurposed the machine as a Steam Big Picture box, (Which sadly won't run on an XBMC Apple TV)
Out of curiosity, what functionality in XBMC do you find particularly worthwhile?
Wider format compatibility - ATV doesn't support AVI, and won't play direct from either UPNP or SMB.