I use the Dlink DSM 320.
I've had it up and running for about a week now.
The setup was very easy, and I was watching Videos and running slide shows with background music in about a 1/2 hour.
I bought the unit primarily because it will handle WAV files. Maybe I am in the minority, but I am a stickler for WAV files. I've done several tests comparing MP3 to WAV, and to my ear the MP3's are not high enough quality for my tastes.
The interface that comes with the unit is fine for photos and video. I've had no trouble playing any files, and my folder structure appears on the screen, which makes it easy to find what I am looking for.
However, it does not work well for sorting my 6K WAV files. My WAV's are all in one folder, and I use MC to sort them out. The playlists work fine, but even sorting through my dozens of playlists takes some time.
Also, since the software gets the track info from an ID3 tag, WAV files show up with your actual file name, and no other info.
The advantage to the MC interface is the ability to use the database to sort my files, and that the info. saved in the tag file also shows up.
The only problem now is I have to switch back and forth between the servers depending on if I am playing music, photos, or video, since MC currently does not support the Photos or Video very well. Hopefully that will change in the future.
A suggestion for Media Center would be to add sorting even within a particular letter. For instance, if I sort my files into six sections, I'll get A-f, G-K, etc. Then I can select a particular letter in that field. It would be nice to be able to sort the letters by Be, Bi, Bm, etc. When you are searching through 1800 artists it would make the process a little more bearable.
To sum up, as with any new technology, it isn't perfect. However, I believe we should see many refinements as these types of units gain more market.
Looking forward to UPnP updates as they arrive.