Go with the Comcast DVR
I am a longtime fan of DVRs. When TiVO and ReplayTV both first came out, I bought one of each. I used both for 30 days, and decided I liked ReplayTV better, so I sold the TiVO to a friend. Since then I've been an avid ReplayTV user and have convinced several friends to go the DVR route.
Until I got digital cable. No longer could I use the built-in tuner on the Replay, I had to use the extremely kludge-y IR blaster to change the channel on the cable box so that the Replay could then record the output. Every so often, the IR blaster wouldn't work and I'd record an hour of C-SPAN or something instead of my show. Also, my TV shows were now going from digital to analog to digital again - resulting in more of a quality loss than before. And finally, more and more of the shows I'm interested in are being scheduled opposite one another (damn network programmers!), so I had to resort to recording one show, and downloading the other.
When Comcast in Boston announced availability of the DVR, I wasn't interested until I saw that it had dual tuners. That alone sold it for me. Then when I got it, and realized that the Comcast DVR simply saves the already-digital broadcast directly to the hard drive (no re-encoding = no quality loss!!!!) I disconnected my ReplayTV entirely. Plus it records HDTV, which the ReplayTV can't do either.
Without a doubt, both TiVO and Replay have more user-features and are friendlier to use than the Comcast DVR. But IMO, all those extra features are trumped by dual tuners, and lossless recording.
A tip: Search on the net and you can find codes to enable 30-second skip in the Comcast DVR 6412 (the most commonly used model). 30-second skip is a godsend. Watch your hourlong drama in 43-44 minutes…