don't know the current status, but ...
xbmc plays the files on your apple tv, and the hardware isn't really up to high def movies that aren't in a natively supported format. your computer only acts a file server, you don't need a computer if you have an NAS.
plex converts the files on a computer running their media server to something the appletv can handle, so you'll get smother playback, but the computer running the media server needs to be pretty robust if you're going to stream more than one thing at a time, or if you plan to use it for processor intensive tasks at the same time.
i use plex on my ipad and iphone all the time, works pretty well