I finally made the jump to Windows Media Center 2 months ago or so and, overall, I am much happier for it. I have had TiVos in my house since 2001 and it has gotten to be that I just never watch live TV anymore. I adore the concept of the DVR and am greatly annoyed when I dont have it at my fingertips.
Except for a few hiccups when I started (some due to me and others due to the fact that I bought the Dell 420 when it just came out, before Dell has fully gotten rid of the kinks), the machine has been working famously for about 2 months now. And it's just great.
There are a bunch of things I don't like about it, but for the most part they revolve around the programmability model. The software itself, out of the box, is absolutely awesome - it is SO much more responsive than TiVo (a Quad processor helps!) and the fact that you can upgrade HDDs and hardware generally is great too. The menu system is also miles ahead of TiVo, not to talk about integration with my home network and the Internet - totally and utterly sweet. The Movie Guide is a real nice touch and very easy to use. So is conflict resolution - I find it much better presented and handled.
The one feature I really miss though is grouped-shows by title ordered by date.