Here's a few reasons from a random web page on why the Comcast DVR Sucks!.
Here are mine:
1. To spell out any show name you have to scroll through the whole alphabet. TiVo does it better.
2. When you pause a program, go do something else, then come back, the show is put back to the beginning. TiVo remembers where you paused the show.
3. You have to guess where the show starts after the commercials, when Fast Forwarding. TiVo does it better with their show preview buffer.
4. HD shows sometimes don't get recorded in full screen. TiVo doesn't record HD.
5. Playback is problematic. Sometimes you get jerky playback and pixelation. TiVo is better.
6. Remote is unresponsive. Sometimes the remote does not function. The TiVo remote is better. Way better.
I'm looking forward to the day that Comcast implements TiVo software on their DVRs.