I found it to be intriguing as well -- and I'm also coming from using dedicated "HTPC's" like the Revo...
I've been pretty impressed with the Apple TV's ability to run XBMC and play the HD content I've thrown at it. Even more impressive to me was the fact that I could play all my 720p Movies and TV shows over 802.11g Wifi. The AppleTV does have Wireless N (dual band) so I'm going to pick up a Wireless N access point to try out 1080p content. In the mean time, I tested out some 1080p content over my gigabit network and it streamed just fine. When the 1080p stuff first starts up it has a bit of jitter but it self corrects and streams pretty smooth from there on out. As you already hinted at, the Apple TV can decode 1080p content but it only outputs at 720p.
Regarding the aTV2 + XBMC setup being "ready for primetime", it really depends on what you are looking for. The ATV2 doesn't have a hard drive (just 8GB internal memory) and doesn't have a USB port, so you'll need content accessible on your network. That being said, I've found that the Apple TV works as a great XBMC "client" in my bedroom. As long as your media is accessible over a good local network and is mostly h.264 encoded, it works as a great device to consume network available content.
I have XBMC setup in my living room running as my central server with all my media served up via a Samba share and the XBMC database hosted (via MySQL) for all my clients to access. One of my favorite parts about it is I can start watching a movie or TV show in the living room, then pause the show and head into the bedroom where I can pick back exactly where I left off.
Yes, you can install plugins and skins. To clarify what kevron was saying, you can install plugins the manual way by copying them into folders like he said. Additionally, since the release of Dharma, many plugins have been moved to the official Dharma Add-On repository. The add-on repository can be accessed directly from within XBMC and makes it really easy to install plugins and skins.
That being said, I use the out-of-the-box skin. It's very responsive -- you can jump from menu item to menu item very quickly, but the animations almost seem as if they are running at a lower framerate or something. On the other hand, I use a number of the video plugins and I've been very impressed at how easy they are to install and how well they work on the ATV2.