Customers are using...

Honestly we don't know what they are using, really!

SSH/FTP/IRC/HTTP/SMTP/NNTP are very common
People are using those without even noticing

They could be trying more exotic stuffs
IPsec, Translators, other transition tools, VoIP
Note all of the current customers are dual-stack sites, otherwise they won't be able to query DNS

Customers do depend on our IPv6 services
it's not a research toy any more