Thomas Hawk's Digital Connection has a great comparison between TiVo and Windows XP Media Center Edition.
Most people at some point upgrade their PC anyways. I know I've been through several computers over my life and at some point I'll be ready to upgrade again. Now if the next time I upgrade my computer, I buy one with Media Center (by the way both Dell and Gateway now ship their PCs with Media Center as the default and it costs no more money than regular XP Home) for my home office, then (since I was getting a new computer anyways) my only expense would be buying the TV tuners and an extender unit. Now this would make the proposition cheaper as well.
While I would love a TiVo, I bought a Windows XP Media Center PC almost a year ago (and I love it). I needed a faster computer anyway, and the premium was minimal. I use it to record TV and it's my main "work" PC (you can use it while it's recording). I can even watch television on it from another location using Orb.
Do you have a TiVo or Media Center? What do you like or dislike about it?