I got a MediaMVP and a WinTV-PVR250 as Christmas present. I had to spend some time learning about video formats, utilities, codecs, subtitles, etc, but now it plays an interesting role in our home life.
The MediaMVP is a small box, like 2 DVD boxes, that goes besides your TV and eventually your audio equipment. It runs a Linux OS and connects to a Windows Service that runs in my home PC using a wired ethernet connection.
You can use it to stream video, audio, internet radio, and to see pictures in the TV. It supports MPEG video and MP3 audio natively and DivX and WMA by decoding it in the server.
Sometimes you find video files that cannot be viewed and you need to convert it using the right tool.
It's really cool and it works well. My kids love it, as I can program the PC to record TV cartoons when they are in school and they can see those hundreds of times. If I were not respectful with copyright laws, I also could download video clips and movies, and my wife would like them.
The software side is also interesting. There is a free (and written in .NET) server for it (http://www.gbpvr.com/) that is really cool. There is also a developer community making linux mods and similar stuff.
However, you cannot compare the software with having a windows box. Windows Media Center is probably better in that regard. But I really did not want to buy a $ 1000 box when I could buy a $89 one.
The main drawback is that it does not have a WiFi connection and it does not support Dolby digital audio. I hope they fix that in the next version, as I plan to buy one more for another TV, as you can have several clients running on the same server.