I own 2 of each of the DSM510 and DSM520 media players. Both are upgraded to latest firmware. Here's what I'm experiencing with them:
DSM510. Nice little box, optical output works. HDMI output works, but needs to be re-set each time unit is turned off and then back on. Leaving the unit on with HDMI active is no good because it never fails to lock up by the next morning. I have to unplug/reboot each morning. So, I leave it connected via the analog plugs and don't have this particular problem. To save space, there is a single 1/8" jack that has video and L/R audio in the same plug. This plug unfortunately does not make too good of contact inside the unit and the video/audio signals don't get a good connection until you "play" around with them for a while. Just make sure no one touches the unit after it's set up and all should be ok. DLINK knows about the problem with the unit and the HDMI output not being saved in the configuration (don't know if they realize it will lock up if left on with HDMI enabled - and the t.v. is subsequently turned off).
One other thing I have found with this is longer moves quit playing about 2 hours into the movie. Movies that play fine on other media players (including the 520) will stop at the same point - giving some kind of server error). However, if you hit play again, it will pick up where it left off. This is with WMP11. The software that ships with either unit is a complete resource hog, and I doubt you'd get more than one unit to stream at a time unless you had a fast dual core system (and then probably no more than two). WMP11 uses around 2-5% resources at most.
DSM520. Does not suffer from the same "long movie freeze" as the 510 suffers from, but it seems to have a few quirks of it's own. The main one being the optical output every few minutes of a movie will drop all audio for an instant. It happens 20-30 times a movie usually, and it's pretty irritating. There is nothing wrong the the movie encoding since it does not happen when my TV is decoding the audio directly from the HDMI plug (also does not happen on the 510). Using a coaxial cable eliminates this problem, but presents another problem if you don't have that input or have a limited number of those inputs (like me) when forced to use coaxial to actually hear the whole movie.
The drop out is not for long - maybe 1/10th of a second.
Don't get me wrong, I love the dlink products I have, but it sure would be nice if the firmware revisions were tested a little more than they seem to be. I would have thought the optical drop-out (which does happen on both 520's that I have) would have been obvious in QC testing.
In conclusion, these types of media players are the future. I just hope the bugs get worked out sooner than later.
DLINK: One thing that would be greatly appreciated in future 520 upgrades is to allow for longer file names to appear on the screen. There seems to be enough room to put quite a few more characters of the file name on the screen. For some reason, the 520 doesn't display near as many characters of the file name as the 510 (but as designed, has much more screen space to do so).