I have several devices for media streaming in my house:
-Windows 7 Media Center thru Aspire Revo 3610
-PS3
-WDTV Live
-xbmc thru soft modded xbox
Content is streamed thru a Buffalo Linkstation Live Media Server via MoCA routers (thanks to the recommendations from this site).
I'd rank the devices in the same order I've posted them above with the Aspire Revo being the best, and the XBox with XBMC last. Overall, they are all good for media streaming. Preferences will come up for different needs.
The Aspire Revo, with Windows 7 has the easiest-to-use set up. I also use Boxee and Hulu Desktop on this HTPC from time to time. It's simple, looks great, and streams well. DVD rips display the original menus and you can enable subtitles and different soundtracks (DTS and 5.1 thru SPDIF is a major plus). I like the Windows Media Center user interface and the flexible options for different programs. Having netflix and internet TV integrated is also another great plus of the Media Center.
The PS3 works good as a stand alone unit, but even better with playon or ps3 media server active on your computer (it does extra encoding of rips). It has SPDIF, so it's perfect for a home theater set up. The user interface is boring and is set up as a file/folder directory.
WDTV Live has all the hardware goodies in a small package: SPDIF sound plus HDMI. It is very similar to the PS3, but without a blu-ray/video game player. Very boring user interface and file/folder directory, but very functional, and not too glitchy.
I ranked the XBox with XBMC last due to its lack of spdif and lack of hdmi. It has a very nice user interface (I'd actually rank the UI second to Windows Media Center). Its very configurable and has a lot of nice web content (e.g. CBS, youtube, apple trailers, etc.). If you need a spare media streaming device for your bedroom, like I did, it is hard to beat. It is low-priced, easy to use, and fits the needs of my room (no SPDIF or HDMI needed). My only other criticism is that it crashes and freezes from time to time.