Also IPv6 was almost entirely unavailable until a few years ago. There's no point in learning how to use ipv6 if your ISP does not support it.
HE.net works and was relatively nice experience to learn how IPv6 work, although both SixXS and HE you still had the disadvantages of tunneling like low speeds, outages etc.
It's nice to test if your application can work on IPv6, but you still won't get benefits of natively using it.