What? I've never heard of IPv6 problems like that at all (and I've had the IPv6 stack enabled since XP!)
Trust me, I've personally had dozens of cases (edging on hundreds pretty soon) where this has been exactly the problem. Given all of our helpdesk, it runs into the hundreds or thousands of cases easily.
Just from last week, in one case a customer with Vista had an absolutely, horrifically crappy internet connection through mobile broadband (3G/UMTS). The lag between anything happening when I took a remote connection was 20 to 30 seconds and webpages were barely loading. After a big struggle, I managed to turn the IPv6 stack off in the local area connection, wireless connection and bluetooth connection in that machine. Reboot, reestablish remote connection and everything ran smooth as silk at maximum speed.
Another case, customer calls tech support several times, can't get email client to work no matter what. When we finally get around to installing Thunderbird and configuring the crap out of it, it establishes connection to server and then freezes to the message "Downloading message 1/500". Turn off IPv6 on all connections, reboot and wham, everything works correctly.
I've personally had several cases where the customer's connection simply will not budge anywhere before you disable IPv6. Trying to get WLAN working is more often wasted effort if it's enabled, though this is not guaranteed.
But in any event it's so prevalent that standard operating procedure at my workplace is that if there's a connection problem on Vista or Windows 7, IPv6 is the first thing that goes after the cables are checked and the line checked. If we file a field case and it turns out it was the customer's equipment, they get the bill, so we have to do it anyway.
I can tell you that it was a complete bitch to troubleshoot this in the early cases, because it was all trial and error, but it was consistently the only thing that fixed otherwise unfixable problems.
The real killer was that remote connection case last week. The IPv6 stack is supposed to be independently set for each network connection, so theoretically they are not supposed to affect each other. But practice has shown that if you leave it enabled in so much as a single network connection item there, it will fuck up every connection in Windows. I've got examples of that too. Disabled IPv6 on local area connection but left it on in the wireless one, the local area connection didn't work and once IPv6 was turned off from the wireless and the machine rebooted, both were a-okay.
I don't know how much of that is due to the fact that 95% or more of our customers use brand computers that come with Windows preinstalled and who knows what OEM crapware on the side. Most people with self-assembled computers don't have problems like this. Some do, but most don't.
But it's a fucking nightmare is what it is. I asked the network core side people about IPv6 and they said our network has some support in the big pipes, but it's completely absent from the DSLAM level and the customer connections. And trying to connect a PC to a modem does not even involve the network at all.