At work recently we decided to implement our own custom forms. The requirement was that we needed to be able to switch out our standard forms with variants so we could do A-B testing. We already make use of Formstack for one off custom forms, so we decided to look into what it would take to utilize FS. As requirements matured we realized FS was not going to work for our needs.
We finally decided on AlpacaJs which is a jQuery plugin for dynamic forms. It allows us to store a standardized JSON form schema in our database and dynamically then load these forms into our site. The library is being actively maintained on GitHub check it out! Alpaca is not without issues, but overall the concept is solid and it's working well for our needs.