I have 2 IPv6 (AYIYA) tunnels from SixXS to my network, both of which are provided by the Nordic ISP Phonera. The tunnels terminate at 2 different PoPs - Copenhagen, Denmark and Oslo, Sweden. This provides some redundancy and has been useful in the past when one of the PoPs has had a problem.
In order to provide reverse DNS resolution for the tunnel subnets, I need to provide the address of my DNS servers. Given that I have 2 DNS servers, 2 tunnels and 2 DNS suffixes (in two different TLDs for resiliency) that gives a total possibility of 8 domains:-
I make use of v6ns.org because the domain is reachable with an IPv6-only resolver, unlike my own domain which is hosted with Gandi as they do not yet support IPv6 glue - even with an out-of-bailiwick server.
However, the zones are currently not reachable by an IPv6-only resolver, because Phonera's DNS servers are only reachable over IPv4 (which serve 8.d.22.214.171.124.0.2.ip6.arpa.). In addition to my IPv6-only name servers, I make use of the free DNS services provided by GratisDNS.dk. Unfortunately, they do not currently support secondary zones with IPv6 masters, so I have to maintain two copies of the zone files - one on my local servers, and one on GratisDNS.
Update: I no longer use v6ns.org because the domain appears to have expired and I have not been able to get in touch with the operator.