Not really using, but being just as curious I bought two Yubikeys to play with, one U2F-only and one Yubikey 4 (with all the bells and whistles except near-field comms).
I am planning to use U2F as a second-factor authentication for administrative privileges etc. The reason to have two is so that I don't get locked out in case one is damaged or lost.
Then I would like to experiment with the Yubikey 4 - it supports a number of challenge-response schemes and has a built-in smartcard plus smart-card reader. So will use pre-existing drivers and other infrastructure. It can even be used to generate key pairs with the private key never leaving the device, so it will be difficult to steal. Of course, without a PIN pad it is still vulnerable to keyloggers, unlike a proper card reader.
(A side note on the NFC-enabled Yubikey Neo, if anyone is considering buying one: At the moment, NFC is only supported on Android. According to Yubico this is because of restrictions in the access model of iOS.)