I've finally got the MythTV setup on the HTPC to the point where it's usable (ie. we can play DVDs, watch TV, play music and watch video files). The last hurdle yesterday was getting mplayer to play video in the correct aspect ratio. I finally figured out that I had to hard-code the screen aspect ratio (1280x768 is 1.666 which is between 4:3 and 16:9).
There's lots of little tweaks still to make, including getting the remote control to work consistently. For now, we just use the wireless keyboard, which is bearable. MythTV is definitely not for the faint-of-heart in terms of setting up though. All up, it's taken me months (on and off, including a period of complete arms-in-the-air resignation) to get going.
I'm glad I persevered though. There's lots of benefits, including time-shifting the TV (a.k.a. pausing), watching downloaded content (e.g. TV shows that won't get airtime here), being able to skip pointless intro screens on DVDs and being able to play some DVDs that won't play in my older DVD player.