"I must, sadly, withdraw my endorsement of yubikey 4 devices (and perhaps all newer yubikeys), as apparently Yubico has replaced all open-source components that made yubikey NEOs so awesome with proprietary closed-source code in Yubikey 4s […]"
https://plus.google.com/+KonstantinRyabitsev/posts/4a7RNxtt7vy
Although I don't use the Yubikey myself, it is makes me quite sad that the developers decided to make one of their products non-free.
Maybe we can convince them to readjust their decision.
Regards
-- egnun