I'm running mythtv at home -- my primary frontend is mythbuntu on a ShuttlePC K45 (the backends are slightly beefier desktops hidden away in the home office).
Peripheral-wise, I've got an ATI remote wonder that works pretty well, but needed some serious configuration in lirc to get it customized. I'm also using a Logitech bluetooth keyboard that was branded for use with the PS3, and some cheap-o logitech controllers for emulated games.
In addition to watching live TV and recorded programs, I can catch shows on Hulu, and I'm running a number of emulators (ePSXe, zsnes, and fceultra).
I highly recomend this setup. One of the reasons I chose MythTV is the all-in-one nature of the solution. The emulators work very well and the PVR functions of mythtv are better than many of the commercial systems I've used in the past.
The Shuttle PC I'm using appears to have been discontinued, so maybe the K4800 would be an economical replacement. Over the next year or so some of the Atom- and Ion-based systems are showing some promise. Personally, I'm waiting until the technology matures a little more and I see more detailed user reviews on teh intarwebz before I go through the headache of finding new frontend hardware.
Interestingly enough, while the topic references emulators on a media center box, I'm also working on a way to get mythtv-stored content streamed to a Pandora (essentially running media center functionality on an emulator box). I've had some moderate success tweaking gmyth's sample programs so they don't run away with all your memory after 10-15 minutes of streaming video.