Overall, I liked the DSM-520. I was happy with the quality of the digital high-definition display and its support for a relatively large number of media formats. Its user-interface was well done, and it did a good job of accommodating foreign media servers.
Where the device fell short was in handling my large MP3 collection. I have thousands of MP3s to which I've added album art, and the DSM-520 failed to play a good deal of them. Hopefully, a firmware update will correct this issue.
Another little quirk that annoyed me was a bright blue LED on the front panel that flashed along with the network traffic. Maybe this would be fine on a switch or router, but it's a clear distraction in an entertainment center. Finally, I wish the device played more of my video files, but with the large number of formats in use, it's a tough problem to solve.
Since I already have a high-definition capable Buffalo LinkTheater in my entertainment center, it's natural for me to make some comparisons. Both handle picture slide shows fine but only the LinkTheater is able to play music that contains album art. Although the LinkTheater has only analog (component) HD output while the DSM-520 has digital, to my untrained eye at least, there is little difference in picture quality.
As far as the ability to play back video files, neither product is a clear winner. Some files that play on the LinkTheater won't play on the DSM-520 and the reverse is true as well. Which brings me to my main complaint with all of these video playback boxes. Manufacturers generally use a dedicated video decoder chip for playing back video, but there just isn't enough standardization for these chips to handle all existing formats.
The only solution so far to this problem seems to be to rely on a full-fledged PC, but that brings in higher cost, power, noise, heat, etc. There is also the transcoding-in-the-server approach that Buffalo uses in its LinkTheater Mini, but then that requires using a PC-based server instead of being able to use UPnP-enabled NASes. Neither of these solutions is great, so I hope that manufacturers will work to find a better way.
The bottom line is that I found the DSM-520 to be a solid product. However if I had to choose between the two, I'd stick with the Buffalo LinkTheater, mainly due to the MP3 / album art issue. But if your media collection is supported and you can handle the little quirks that I found, I think you'll find the DSM-520 to be a good HD media player.