Works as advertised, no issues.
The only drawback is that - despite the tooltip - users are confused about the new field on the log-in screen. It doesn't even help to inform them in their sign-up emails (because after some weeks they forgot, or they use one login for a company, or they don't read the small print ...)
A great enhancement would therefore be to have the field by default hidden (reveal with checkbox) or to make it (like Google, Dropbox etc.) in two steps: only users who have GA enabled see the field for the code in the next step.