
I used to swear by SageTV up and down. I was a very early adopter of this software and was promised "Free Upgrades" when I originally purchased it. Apparently "Free Upgrades" only applied until version 5.0+ was released, then it started costing money for every major revision.
It's still good software, however I have a problem with advertising "Free Upgrades" and then skipping out on that claim. I stopped using them on that alone. (Don't lecture me on how they need to get paid for their work, I get that. Don't advertise "free upgrades" if you aren't going to follow up with that claim.)
I have since moved to XBMC http://xbmc.org/ running on an ASRock ION 330 plugged straight into my home theater receiver and it plays 1080p pretty well, 720p flawlessly. The only drawback of XBMC is that it doesn't record (it was never meant to). Its playback capabilities are unmatched in my opinion (mplayer backend, if mplayer can play it, so can xbmc). It's crazy easy to install and use. I use the genuine Microsoft MCE usb remote control which is supported out of the box with no special LIRC knowledge.
I use MythTV with a Hauppage HD-PVR back end for video recording, and although I believe there is a MythTV FrontEnd addon for XBMC, I simply share my recordings directory via CIFS and let XBMC mount the share directly for playback. I like the seperate MythTV backend because I have two XBMC/ASRock installations and both can then read from the same source for playback in either room.
XBMC: Free (Please Donate, its really that awesome)
MythTV: Free (Please Donate, its really that awesome)
ASRock ION 330: About $350 on newegg
Hauppage HD-PVR: About $200 on newegg
panic: kernel trap (ignored)