I think the first time i saw IPv6 on the Internet was in the late nineties on IRC, when it was cool to have a hostname like 3ffe:1337::dead:babe. A few years later i tried out freenet6 but stopped using it, when most of the IRC servers k-lined freenet6 because of script kiddies abusing it. IRC was and maybe still is the only IPv6 killer application.
Since 2005 i have been using sixxs.net and started using IPv6 on my LAN. If i can trust the gif-interface counters my sixxs.net tunnel currently transfers about 70MB/day, while the IPv4 interface claims to transfer about 300MB/day.
I recently converted not only this webserver but also the kde-freebsd Teams’ development host to ipv6only. I did not expect this to be a big deal, as all people having accounts on it are unix geeks. I was quite surprised that more than half of the users did not have any ipv6 experience.
Another datapoint is freefall.freebsd.org, a system accessed by unix hackers only. According to netstat there are currently 43 tcp4 ssh sessions and only 10 tcp6 sessions,
Even worse, of these 10 sessions three are freebsd.org-LAN sessions and 2 are from my systems.
So if you have something interesting to offer on the internet, shut down your IPv4 interface.