I jailbroke my new AppleTV 2 tonight using Greepois0n RC 6.1 on my Mac using a Kindle micro USB cable. Took me 2 tries to get the timing down, but was very easy. Updated Cydia, then installed XBMC and started playing movies - super easy. Plex isn't in the "app store" yet, so I had to manually grab it. In OS X's Terminal, the commands I used in order are: (anything in bold, paste in - this is for iOS 4.2+)
ssh root@192.168.1.200 (insert your ATV2's IP here)
RSA fingerprint blah blah blah:
yes
password:
alpine
echo "deb http://awkwardtv.dyndns.tv/ stable main" > /etc/apt/sources.list.d/awkwardtv.list
apt-get update
apt-get upgrade
apt-get install com.plex.client-plugin
killall AppleTV
My Plex Media Server popped right up, scraped artwork et. all. Really simple config, blends right into the GUI and gives you options for Movies, TV Shows, etc. This gave me Plex 0.6.7, but I think .8 is out now. Looking more into this later tonight.
XBMC: More features and options, more polished GUI. 1/2-second lag in the GUI, slightly annoying but not a dealbreaker. Launches as it's own app, so it takes over the stock ATV menu. Video playback is really great.
Plex: Integrated right into the ATV GUI, talks to the media server for streaming & scraped data, great transport controls. Ability to combine multiple servers and add remote servers. Video transcoding quality not as good as streaming, but pretty dang good for transcoding (95% as good as the original I'd say - but I could tell there was a bit of rastering, aliasing, and slight color loss, like in Avatar). The GUI looked aliased, which bugged me. No nice loading percentage like on XBMC; it just spins until it loads the video. I'd like to see viewing options instead of just a list with the thumbnail on the left - personally I really like the "wall of movie covers" like XBMC has an option for.
I am a huge XBMC fan, and also of Plex for Mac. If Plex for ATV2 can add native streaming and clean up the GUI a bit, that will be the ticket for me - seamless integration into the ATV's GUI and seamless integration into my network library. Done and done. XBMC proves that streaming is possible and Plex proves that GUI integration is doable, now I just want to see them combined!