You'll spend more in man-hours and probably more on equipment getting any of the linux pvr solutions running than just purchasing TiVo.
The advantages to using a linux solution are in scalability (yes, even taking into account TiVo hacking.)
For the average user, it's probably not worth the effort, but if your library is huge, you want to be able to watch from multiple computers/tvs and perhaps record multiple programs at the same time, it might be worth it.
If you're seriously considering MythTV, keep in mind that some kind folks maintain pre-packaged binaries for different popular distros. you can find links to them on the mythtv.org documentation section I believe.
