Hrm. From what I have seen, Windows has pretty good IPv6 support as a client at least. I haven't tried to use it as a router/firewall or anything like that though. I probably wouldn't try that anyway even for IPv4.
I know a bunch of people are using Window for 6in4. :shrug:
Windows also has Teredo built in, which makes it pretty easy for just about anyone to get IPv6. Sure, it's inefficient, but generally all you need to do is turn it on and voila, you have v6.
Windows still lacks in several major areas.
Like the fact in XP and 2003 even you can't modify the IPv6 settings through the GUI, it's all command line crap. On top of that many versions of Windows don't allow pure IPv6 as far as I remember, you could for example setup a test machine on an IPv6 network and disable the IPv4 stack in XP and you wouldn't be able to do much of anything. It still relies on IPv4 for DNS. At least that was my results with XP SP3 and 2003 both.
Remote Desktop and similar things, like many functions of IIS are not IPv6 aware as well. Microsoft has a LONG way to go for IPv6 to be fully functional IMO. Windows 7 isn't bad so far, and from what I understand 2008 works fairly well with it. If you consider the age of IPv6 and how long it's been in development I would say MS really waited as long as they could to start supporting it.
Again, I understand many of the reasons WHY they waited, I'm just saying they are very far behind NIX systems in their support. I'm a Windows desktop person, not some MS hater so I'm not trying to just rant about them or anything. Only pointing out I wish they would have put more work into IPv6 earlier. I don't believe until Windows 7 and 2008 Server are standard will Windows really be viable for IPv6 in most environments.