I installed MythTV because the interface is designed to be used with just a few keyboard commands. At first, the goal was to replace a dead TiVo with it, but we quickly realized that we had plenty to watch without capturing TV. So now we just use the streaming features of it, and access DVD images and downloads from a closet server.
Streaming in MythTV is a bit weak on this slightly older setup (about a year old) - the flash player loves to steal input focus, so I have to go over and click to get the remote working again. I'm hoping that irritation is fixed now. Browsing for streaming content isn't so great in the MythTV interface, might as well launch a browser really, you need to use a keyboard to accomplish anything - and the OSK is slow and clumsy. Hulu isn't working on this slightly older Ubuntu, though it is fine in my desktop's 11.04 (probably will update the TV box soon). Netflix actively refuses to work in Linux.
Input devices seem to be a weak point to me if you just want to plug in a random PC. Get yourself a remote that works like a keyboard. Personally, being a nerd, I used a micro-controller (Atmel USB capable AVR) and an IR receiver to fake a USB PC keyboard with my Sony TV remote - so everything (volume, TV power, full MythTV control) is available with one simple remote.
So, fair warning, you'll probably have an all around easier experience by buying a PC remote control and installing Windows Media Center on it, if you can stomach running such a thing.