We just recently upgraded to 5.2 and as soon as we did I enabled the Time Based Token module. I don't know if any clients are using it yet because we haven't sent out any e-mail or posted anything regarding it yet but we hope to have an announcement e-mail going out in the very near future for other things we want clients to test.
We have a similar system to the 2Factor built in on the back-end for our Administrators and Technical staff, but we do not require our clients use this system.
We started using it (Google Authenticator w/ SMS) via third-party addon some time ago, with all back-end staff required to use it. We don't have any clients using it at the moment, however it was never announced as being available for use.