Comcast has similar performance in all the cities I've lived in as an adult (Los Angeles area, Silicon Valley, Seattle, small towns on the west side of Puget Sound). Certainly not what I expected given all the complaining I see on the net about Comcast and YouTube performance.
Comcast gives many things about which to complain, but I've never had an issue with Youtube on them. What I've been shocked by in the past is the disparity between youtube on my comcast connection and youtube on people's TimeWarner connection when I've visited, where it has been completely abysmal in the past. Like, 1990s RealPlayer quality, not only because you have to turn it down to 144p just to play, but also because it still stutters horribly even at that resolution.
I feel like most of the "one weird DNS trick to fix youtube" posts have been about timewarner, though I don't know if my memory is accurate there.
Notably TimeWarner looks fine on these graphs (at least in a few places where I've had friends complain about it in the past), so maybe they've fixed things.