The YubiKey NEO has been on my keychain for about two years.

The entire NFC function vanished after the first few months.  It still works as a USB YubiKey (same as 4?) but the NFC is totally nonresponsive.

Other than that, works reliable and with most programs.  When approaching new Windows7 machines, it does spend an inordinate amount of time loading drivers before it can be used.  This is improved by using their customizer tool and deleting all the protocols you don't intend to use -- fewer drivers to install.

It has never failed to authenticate over USB, but I paid extra for the NFC function and it works with no phones or readers at all.