I have Comcast Xfinity (stupid name if you ask me) but I actually have had no problems. It's pretty damn fast here in Connecticut. My only gripe is that I wish they would tell you when they're performing maintenance... I hate playing Live only to find that my connection has gone to -blam!-. As for their other services (I have internet, TV and voice), I just wish they would update the software for TV, it's extremely outdated and slow. Finally, their customer service has greatly improved over the past year. It used to be that you'd have to wait nearly an hour for help (due to "higher than normal call volume") but it seems as though that problem has been rectified. Every time I've called for help in the recent past, my problem has been solved quickly and the solution has worked.
By the way, those of you who have Comcast that haven't updated your modem should definitely do so. The new modem is much better and the upgrade is free.
[Edited on 12.26.2012 11:06 AM PST]