Google publishes a map of what percentage of users are accessing them via IPV6.[1]. The numbers aren't great. The US is around 53%, UK 43%, Brazil 45%, France 74%, India 68%. And many countries much lower. I believe the setup is such that if the end user's IPV6 was functional, they get counted.

[1] https://www.google.com/intl/en/ipv6/statistics.html#tab=per-...