I'm 95% done with my new Media PC.
I previously had used a Hauppauge MediaMVP with the custom SageTV firmware (pretty much reprograms it as a Sage terminal). Nice piece of hardware for $100 or so. Did the job. One problem was that it couldn't show streaming content for fairly obvious reasons. But great on downloaded or archived TV shows. The other problem was that the interface was a bit sluggish and strange things would happen like not being able to rewind. Technologically, its amazing it works at all. But just not a premium experience. The other problem is that the TV card in the Sage "host" computer is fairly low quality and the computer itself is not reliable enough for an always-on appliance. So it didn't replace the Tivo.
I built the Media PC from scratch. Shuttle case (very elegant and incredibly dense fit). Core Duo 4300. 1TB of HD.
Picture quality is great though I'm not thrilled by the resolution. Have to switch between "Zoom" aspect for watching TV and "Full" aspect for using the computer. The onboard card doesn't support custom resolutions. Probably off to buy a cheap card. Accidentally, didn't get the SPDIF interface for long-term sound (on order).
I was surprised that Vista doesn't support drive mirroring at the OS level. I was surprised how quickly it installed. I was surprised how quickly it reinstalled after I hit the "disappearing control panel" problem, rendering my system useless, within 5 hours. Apparently, there's a nasty interaction between some drivers and Vista that makes the control panel inaccessible. I'm not sure whether it was updated Hauppauge WinTVPVR drivers or Powerstrip, but I'm not touching either again.
I'm surprised that Media Center doesn't allow and automatically consolidate multiple data locations (since without mirroring, I now have 950GB of media space). You can have media in multiple locations, but you can only record to a single location. Doesn't seem like a hard problem. The Media Center interface isn't as good as the Tivo being replaced (hopefully), but its good enough. They were definitely too cute on their interface design.
The biggest surprise was that my new $30 DVD writer was much, much better than my 4-year-old $30 DVD reader at ripping CDs. Several "unrippable" CDs had clean reads. And it rips fast.