I have been hunting for the perfect network media streamer. I first started with a Popcorn Hour A-100 which played pretty much everything I threw at it but the user interface was raw to say the least. I was moderately happy with the Boxee Box but still found the software buggy and playback to be hit-or-miss. I even tried hacking an Apple TV, Xbox 360 and PS3 but all had their issues. I finally broke down and bought the base model Mac Mini 2.3Ghz i5 and installed Plex. With the Mini's power, it played back 1080p .mkv files streamed over gig ethernet without a hick-up. Combined with Plex's amazing server/client model, I could control my home theater library from my iPad, iPhone and playback video on my TV or my idevice. I paired the Mini with a Lenovo N5902 for couch control and a Xbox 360 controller receiver for emulator games.
The Mac Mini is the PERFECT home theater PC for me blending in seamlessly with the rest of my home theater equipment.