I've used Auth0 on a few projects; I also teach and have assisted students who've used it on their projects. As a service it's great. It combines a myriad of identity providers so you don't have to coordinate them all; they have a pre-built widget if you don't want to create your own login form; and their API allows you to handle authentication on the server if you'd rather do that. The only fault I've found is that their documentation, while it looks really good, is lacking in some ways. As an experienced developer I haven't had many issues other than one inaccuracy, but students of mine have struggled to implement things based on the docs. I'd be happy to answer any questions people have about it though.