I've have both, but dropped the subscription on my TiVo. I have a series 2 Tivo and the Comcast HD DVR.

TiVO (Specific to the model I have)
Pros
Intuitive interface
good search engine (Titles, key words, etc.)
Media center allows you to stream photos, music and save recordings to your PC.

Cons
Since it has to control the DVR, there is a lag changing channels, and sometimes problems recording shows.
Single Tuner (you can't watch a live show and record another)
No HD
More expensive monthly service, or you can do the one time fee.
Analog audio only

Comcast DVR
Pros
Dual tuners
Since the tuners are built in, the only problems recording is when the guide is wrong.
Digital Audio out
High Def and records in High Def


Cons
Search engine not as good
Interface not as intuitive.
Can't stream music or view photos from PC.


After typing this I went to TiVO.com and it looks like they have a dual tuner DVR and a HD DVR, but the HD unit is expensive and the service is a lot too. The DT unit has limitations on what you can record when using both tuners.

-Jeff