Comcast's IPv6 network for content from Netflix or YouTube is actually better than using IPv4. A while back I set up an IPv6 only machine just to see the difference, and it is night and day.

That being said, their network still leaves something to be desired, the IPv6 routes taken to get to the same IPv6/IPv4 host can sometimes be circuitous and I have noticed that they have a higher latency too. So there are upsides and downsides, but I hope it can only get better with time!