I switched from DirecTV to Comcast in the Atlanta area about 2 months ago. Believe it or not, the SD channels are much, much better on Comcast. I was pleasantly surprised with how good they are. DirecTV has been stealing bits with an increasing rate the past few years and the picture quality on DirecTV has suffered. It happened gradually.
So you know, I compared the SD channels on Comcast to DirecTV on a 60" Sony SXRD HDTV. Comcast blew DirecTV away. And the SD channels look great on our SD TV's, much better than DirecTV did on those too.
While I did not have the DirecTV HD package, I've heard from others that it's OK, but not spectacular.
For HD, I have an OTA antenna, and I also get the Comcast HD package. You really don't get much for the extra $$$ on the Comcast HD package. The locals are on the cable and are in the clear, by FCC regulation (except for WUPA, don't know why) so you'll get them even if you don't subscribe to the Comcast HD package. I've compared the HD locals on Comcast to the OTA HD and there is some degradation on Comcast, but it's not bad. HBO and Showtime HD look great, as does DiscoveryHD. INHD is OK too.
The past few days Comcast has been having some problem with some of their HD channels (806, 809, 880). They are breaking up badly. It was fixed on Monday, but just noticed they were messed up again today. Don't know if it's weather related...I tend to doubt that. But the analog versions of those channels are still there, so I can suffer through that.
I am very pleased with OnDemand. Didn't realize that most of what if offers comes along with the package we subscribe to (Digital Silver). So most of OnDemand selections are available at no additional charge. Not a lot of it is in HD, though, perhaps just 5 or 6 movies (the free OnDemand).
We made the switch for several reasons, including, like you, using their Internet service. I plan to switch this month from DSL to Comcast internet. And we'll be dropping our home phone, instead using our cellphones exclisvely.
Overall I am pleased with Comcast. I changed from cable to DirecTV about 7 years ago because the cable quality was really bad. But they've rebuilt the system and their picture quality now exceed that of DirecTV.
BTW, we found that Comcast was price competitive with DirecTV, within $5 or so. And we're using the Comcast/Motorola DCT-3412 HD-DVR. It's not TiVo, and we miss our DirecTiVo. But for the extra monthly fee with Comcast, if it fails they will replace it. So far it's been reliable and we are really pleased with the quality from this HD-DVR (via HDMI).
Finally, I forgot how easy it was to distribute cable through the house. Even HD. We don't have a set-top box on our upstairs 2nd HDTV, but we essentially use it for the locals anyways and they are in the clear in HD via cable anyways. If we miss the other channels, maybe we'll pay the extra and get a set-top box for the 2nd TV, but right now we feel we don't need it.
Just my 2-cents.