Just to add to the RaspBerry Pi version 1 usability..... I recently downloaded and wrote SD Card images for both the latest OSMC and OpenElec (both linked from the RaspBerry Pi web site) and they will play movies right off an attached USB stick, no problem. Seems the codec support is good enough to handle most of my media (AVI, MKV, MP4). No skips or frame drops. I played larger 1280p type files too! I expected slow frame rates or problems with seeking through scenes but it forwarded and reversed without any issues.
That *was not* the case originally a few years ago when I first got my RasPi and tried playing their "Big Buck Bunny" video using the mpeg player in the Linux distro at the time. It was choppy, and then I heard they were going to make you pay for codecs because they were not GPL or licensed or something. I was worried I would never be able to play smooth videos on the RasPi. Obviously it was not the hardware.... It was the software. It was not optimized or the drivers were not tweaked properly.
But as you can see, the amount of support for the RasPi community and many talented people were able to customize and create images for everything from a number of Linux, RISC OS and other distributions, several MAME-type SD Cards.... All free and easily available for download and easy to write to an SD Card. Will the $9 computer have the same support? We just don't know. Time will tell.