Paul has a great write up on MCE and Tivo. I ditched my Tivo 2 years ago for my custom built MCE box. In that time period I've been dogfooding (running alpha and beta builds)various versions of Media Center about 75% of that time. 1.0 had a lot of little gotchas and issues, especially if you built one yourself. My machine runs 24/7 and has rarely missed recordings and only crashed once in 2 years.
Today the product is a lot more stable and the team is doing some amazing work right now (can't talk about it yet). Needless to day, MCE gets better and better every day (literally).
If you want something that doesn't just record TV (for me the Music, Photo, Video, Napster and Newsgator 10 foot experiences are just superb). And mine passed WAF (Wife Acceptance Factor) with flying colors.