I got the DNS-320 a couple of weeks ago. Installed 2 1TB Seagate HDs Raid 1. Enabled UPNP and immediately drive was available on my Samsung DVD player CD5500. The builtin P2P works fine, nothing fancy but gets the job done. AjaxExplorer is basic but can be upgraded if you like, I havent bothered.
Using builtin UPNP I can watch steaming 1080p highdef movies (wired) with no issues. Unfortunately, there is some restriction on the types of movies the Samsung will play and .MKV has to be renamed to .MP4. Subtitles dont work so I tried Twonkyserver but that doesnt play .MP4/.MKV on my Samsung. Of course Fun plug install first using these instructions... http://www.shadowandy.net/2010/11/dns-320-making-fun_plug-and-old-packages-work.htm.
Now working on getting another device to stream this content to.. PS3, custom XMBC build, not sure yet...