My local ISP has what may be the worst DNS servers in the history of the Internet. Back when I used them, things would randomly go south with certain websites being inaccessible. Each time I would call tech support, it seemed they would blame their DNS servers. I decided to fix that and moved to Google Public DNS long ago and have never had issues with resolving domains.
I have wondered on more than one occasion how many people actually use Google's DNS servers. Google has offered the answer to that question, and answer is a ton. Google reports that it serves an average of over 70 billion public DNS requests each day. Google says that its Public DNS is particularly popular with international users with about 70% of its traffic coming from outside the US.
Google also notes that it has added Google Public DNS servers in Australia, India, Japan, and Nigeria. It's hard to imagine 70 million daily requests and the number of servers it would take to handle the workload. Google also notes it is taking steps to support IPv6 with new addresses of 2001:4860:4860::8888 and 2001:4860:4860::8844 along with 184.108.40.206 and 220.127.116.11.