I have used a number of forms and quiz apps, such as Google forms, Zoho, Cognito and the like, and have found Formly a pleasant surprise. My principal use is training course and marketing quizzes. It supports a wide variety of question types all of which can be easily combined in any quiz. That's not so easy to do in some other apps.

It is easy to use with a minimal learning curve, although, my previous experiences with similar apps may have helped, because as I comment below documentation is minimal. Quiz creation is really quick, much faster than those other apps e.g. simply pasting a bunch of answers as a block, instead of entering them individually is a huge time saver. Similarly changing layouts is effortless. It is also deceptively powerful and flexible in branching, calculating, use of variables, displaying scores (with graphics) etc. Again, the implementation is elegant and deceptively simple.

If I have a criticism, it is, as other have noted, that documentation is minimal. Fortunately, I had a session with Ash that was particularly valuable, as I learned of a number of features I hadn't known about and I doubt I would have discovered them. Particularly so, as the UI is very uncluttered (a feature) and so some functions are not obvious.