Comcast also supports 6RD and 6to4 servers, so even if you don't have dual-stack, you can get on the IPv6 bandwagon.
6to4 should "just work", but 6to4 itself has some known issues with some kinds of routing (the IPv6 prefix doesn't have a routable prefix, so not everyone you can see can see you).
Their 6RD servers are few and far between, and that gives bad performance, but it work correctly. You just need to configure your connection properly for 6RD to their 6RD border router.
Windows or Mac OSX directly connected to the internet should work fine. You shouldn't even need to configure anything.
If you have a home router, it probably doesn't support IPv6, but you might be able to use DD-WRT (www.dd-wrt.org) or other replacement firmware that does. I do this, and it works fine
Neither are as nice as native dual-stack, but Comcast has upgraded their equipment for it in only in a few cities,and it also requires your cable modem to be DOCSYS 3.0.
Overall,
I have found right now that using HE's tunnelbroker is better for performance than Comcast's 6RD or 6to4.