Duo seems to have had lots of success, selling into higher education. To add to the other schools mentioned in this thread, Oregon State, and Stanford both use it too. And lots of schools are starting class today, which might be what contributed to their downtime.

To be honest, their useR experience is pretty slick compared to most two factor solutions. I am happy for them, (and their users) that they have been successful, selling into large organizations – good user experience usually isn’t found in large enterprise software due in part to the principal agent problem.