Upgrading the TV in our bedroom set off a chain reaction causing us to have to upgrade to an HD DVR. I didn't really want to spent more money buying a new DVR from Tivo, so though I could save money just going with a Comcast HD DVR. Comcast said it would be 9.95 (actually one person said 8 the other said 9.95), which would save me about 4 dollars a month vs Tivo. But am I in for a surprise on my Comcast bill; will I get charged additional fees? I already discovered Comcast charges you a 2 dollar fee any time you make a change to your service (and I've had to make 3 changes this month before getting a setup that works). I really don't need all the frills Tivo comes with, just something to record shows that's compatible with my HD TV and an HD cable box. Tivo was nice enough to put my account on hold for 2 months while I try out Comcast. So if I'm in for hidden fees from Comcast and I'm not saving money, I might as well just go back to Tivo.
Tivo is just plain better and easier to use. The WAF (wife acceptance factor) is very low with the Comcast DVR. Last time I saw it, the GUI was horrible and not very intuitive. So if the price difference is just a couple bucks, Tivo is a no brainer for me. That said, I'm not using either. I'm using BeyondTV because there is no monthly fee and the GUI is almost as good as Tivo. If you happen to have a spare PC around and a Logitech universal remote, it's $40 for the software ...and once setup correctly, the WAF is high.
I've had both. I don't think there are too many hidden fees...probably just a buck for tax. Tivo is better for me now because I can keep the stuff permanently and make DVDs.