I have a WDTV streaming media player and I can stream HD movies from the DNS-320 network shares. I can also stream photos and music from network shares.
I acknowledge that the built-in UPnP server is awful, but I can also stream movies and videos via dlna to my wdtv and to my samsung galaxy ace. Moreover, an app called 2Player allows me to select the music tracks I have on the DNS-320 and play them on the WDTV, via dlna. I can switch songs from every corner of my house. With other apps, I can stream my whole photo collection to my cell phone or tablet. I had also streamed HD video to a xoom tablet, and I can actually choose a movie from my entire collection and play it on a tablet.
Granted, the firmware has its flaws and is perfectible. And I had experienced video streaming issues. But, so far, I have been using it for more than six months mainly for streaming.
If you have problems, do not hesitate to ask.