Hi,
I don't have any kind of technical knowledge to offer on HD over cable, but I do have some experience with Charter Communications.
Check what your local Charter system offers in terms of programming HD. I had Charter HD cable in Greenville, SC for about a month before recently switching to VOOM.
Obviously in a different area the service may be very different, but in my area Charter's HD offerings were minimal. Only one local HD station (despite the fact that I can now pull in 15+ subchannels of digital programming OTA), HDNet, HDNet Movies, ESPNHD, Braves HD (Atlanta Braves games), and then the HD versions of HBO East and Showtime East if you subscribed to those premium services. The HD picture did look very good, with only occasional pixelling.
On the rest of the cable service, only channels above 100 were digital, so most of the SDTV programming that is digital on VOOM was actually analog on Charter's so-called "digital" cable. It was not very good picture quality. I know others are not terribly happy with SDTV PQ on some of VOOM's channels, but compared to what I was getting on Charter I see a significant improvement.
I had problems with On-demand services with Charter. They hardly ever worked, and when a Tech came out to look at my cable-box, etc he said that the problem wasn't on my end, but that they'd had problems with implementing the service.
When I called to (politely) report problems the CSRs were usually rude and very dismissive. I'm not surprised to hear that in my area Charter has extremely low ratings on customer service surveys. I had Time Warner when I lived in North Carolina and found them to be completely different: responsive and reliable, but YMMV.
I was very happy to return my cable box to Charter.
Best wishes,
CDH.