I've used several media pc/streaming options including Original Apple TV, XBMC, Windows MCE7 and Boxee. While the limitation of video playback types sucks with the "Out of the Box" Apple TV software, the simplicity of it just working is nice. My wife hated all the different media center solutions I came up with and just wanted the Apple TV back. My biggest problem with that was the original Apple TV hardware couldn't handle my Blu Ray encodes.
Now with that in mind, the new Apple TV hardware handles all my encodes flawlessly and the picture quality is better then what I was getting from several different configurations. Also, the response is snappy in the menus and with chapter skips and my wife can easily navigate to the movies and doesn't have to worry about any weird work arounds or having to reboot the media center to get the movie to play.
I have a media server running windows 2008 at the moment and will likely install WHS Veil when the final release is made so running itunes as a service all the time doesn't bother me and I have the process scripted to add new encodes to the iTunes library but I understand the pain of people who want a cheap NAS and don't want to have a computer running all the time to stream the music. Hopefully, Apple will release some iTune streaming alternatives for Time Capsule or someone might come up with a linux based server app that can run on those NAS Appliances.
I am very happy so far with the new ATV and Can't wait to see what Apps come out for it or options that become available via Jail Breaks.