I originally had Comcast Digital Cable in Baltimore and 2 years ago switched to Directv. My reason for switching was due to Comcast's poor service and price increases. Several times the cable would go out, and Comcast would send a signal to the digital box and couldn't find the problem. They wanted me to wait for someone to come out and look at the problem and this meant that I had to stay home and wait. Most of the time it cleared up on its own in a few days, without a service man even coming out. After several occurances I finally switched.
Since then, I have had no problems with Directv, other than the picture going out for 5-10 minutes during heavy storms. At least I can expect that though.
The picture from Directv is more consistent between channels, but to me, most standard TV channels looked better through cable than Directv. Directv just has a blurry look to most channels, some are better than others though. I have a 51" TV, but do not yet have HD hooked up because I'm waiting for the HD boxes to go down in price.
I was paying nearly $70/mo for Digital cable, and now pay only $45/mo and that includes Tivo.