AFAIK I have Comcast and Netflix. Most of the time when I watch Netflix it wavers between 280SD and 480SD. On a good day I get 720HD. On a bad day - which is all too frequent - I get 240SD. Anything from 280SD and below looks like crap on a modern HDTV.
And yet, at the same time I can run a bandwidth test which shows I get great bandwidth (speedtest.net, local test server.)
I suppose the bottleneck could be beyond the portion of the network between my home and the local speedtest.net server. Or Comcast is throttling Netflix. Either way, Comcast has a vested interest in not solving the problem.