Much to my surprise, Comcast rolled out IPv6. Somewhat better than VirginMedia in the UK, who seemed to be kicking the can down the road as much as it could.
I did set the cable box into modem-only mode and have my own router (running OpenWRT) as my primary gateway. This was after Comcast did an 'upgrade' on the old box which broke Wifi and also reset it to default and I lost all my firewall settings. Now they can't. The IPv6 defaults to 'safe' because unless I make the effort to add a traffic rule, it won't pass IPv6 packets from the WAN to the LAN, so it's no worse than IPv4 and a NAT router with no port forwarding. That lets you use IPv6 outbound but still only be accessible on IPv4 if you've enabled any inbound stuff.
The only irritating thing is that the IPv6 prefix changes occasionally, I guess it's the equivalent of re-segmenting the network and changing everyone's dynamic IPv4 address.