I bought this as I was looking for a solution to replace playing media through my Xbox (Having to transcode was becoming a chore) and after a little research settled on this.
Quite simply it is amazing. It plays pretty much everything you would want it to (1080p MKV, DTS sound, mp4, Avi, ISO dvd images).
Its simple to set up and you'll be watching media from attached storage in minutes.
Connecting the device to the internet and your network increases the functionality immediately with the ability to watch YouTube, and to stream files from PC's on your network.
Western Digital seem to be providing a lot of support for the device with a number of official firmware updates being released in the time I've owned bringing added functionality (Facebook, mediafly, deezer, DVD menus in ISO images) and bug fixes (Not that I ever experienced any).
There is also a large group of individuals who have unlocked the maximum potential out of this little box and released their own custom firmware, which is as easy to install, and with that comes EVEN MORE functionality: YouTube HD, Apple Trailers HD, BBC radio podcasts, weather updates and customisable displays (google Moviesheets).
There are many forums providing support and advice in how to improve your box and although many are over my head, everyone is in the main helpful and i've been able to update my device significantly.
In short, buy this box, today!