I think you have to separate out Music and Video.
For Music, I want 2 way remote and integration into Itunes. Sonos does support that, Apple TV as well. I do not want to deal with mouse and keyboard in my living room which you have to with Mini Mac/ Imac based on my experience.
For Video, I don't like to use Itunes as it stands. I like the idea of having an automated library, meaning base don file name it should pull movie trailer, review and DVD cover for me. There are quite a few applications that do that. Also I like integration with streamaing services like Youtube or Netflix or Hulu. I have my ATV hacked to run a software like that, however it does not have enough processor power to deal with much content besides whats coming from Itunes.
Most products mentioned here offer only simple navigation, they miss the automated trailer and cover functionality for the Library.
My setup, I run ATV with Boxee Media Center and a NAS to handle my video. Down the line I will use this for Music and my Ipod Touch as a 2 way remote. There should be an upgraded ATV coming soon, which I am sure will provide more processor power and it should not take long to install applications like Boxee.
Oh, my ATV with Boxee is controlled through Beo 4. I basically have a Kaleidescape installation on the cheap. Disadvantage is that I have to compress the DVD files and convert them to MP4.
JK