FormIt has been designed to work well with Revit. To learn more about the basics, refer to this section in the FormIt Primer tutorial.
If you have a project started, you can send geometry from Revit to FormIt. This is useful for doing quick study models and then sending geometry back to Revit. Here is a video demonstration:
FormIt can be used to create content for use in Revit families. Here is a video demonstration: