We successfully used CB with Formit to provide users with form builder, including single and two-column form layout, text fields, textareas, radio groups, checkbox groups, file upload fields, dropdowns, email notifications, autoresponders etc. Initial setup is quite complicated, you need to understand quite well CB, Formit and MODX itself. But in the end of the day, customer is happy
I was also dabbling with that idea some time ago, but then decided to extend my contact-form-element to a “Multi-Purpose” Contact-Form, which is a “very light” Form-Builder: