I am in the "process" of switching, meaning that Comcast doesn't quite have their act together in my neighborhood. I have the Comcast HD receiver, but the HD/DVR I want (and NEED, as I'm used to NOT watching commercials!) is not yet available in my North SF Bay area (I keep hearing "one to two weeks" but that has been going on for almost two months!). So I still have DirecTV service with the Ultimate TV DVR as well...but that's getting kind of expensive!
From what I've seen so far on the Comcast digital HD channels, the picture is great (better than the non-HD satellite picture). But the non-digital Comcast channels (numbers <100) look like crap coming out of the digital receiver! I've had to split the cable signal and run one cable into the HD cable box, and the other into the non-digital antenna input on my TV, then the lower channels look "just OK." I guess someday, all the Comcast channels will be digital....?