Revit families are becoming more common and easier to stumble across. Website such as Autodesk Seek and Revit City have a great collection of families, and many manufacturers are starting to create their own families for their products. So what happens after you exhaust all these options, but still the only thing you can find is a 3D ACAD family?
3D Autocad familes can be used in Revit models, however it is usually best to use discretion when using them, and only go that route after all other options have been exhausted (including creating your own family). In a few instances it has been necessary to use an ACAD model, one that most recently comes to mind is the waterslide and features of an indoor waterpark. Because of the complexity of the waterslide (extrusions that rotate about all 3 axis') it was necessary to use the 3D acad model provided by the manufacturer.
You can usually go about placing the 3D ACAD model in a blank Revit family template and saving as a revit family. However, the 3D ACAD still remains natively a ACAD file, despite loading it into a revit family. (just like an imported ACAD floor plan is still ACAD). Since the geometry of the family was not created with Revit, (such as extrusions, sweeps, etc) you cannot assign sub-categories to the object. This can pose some issues when trying to give say a medical gas reglet a subcategory under it's master category. A category can still be assigned to the family, but the object cannot have a subcategory, making it a little more difficult to congrol graphically in you view.
To ensure there are no issues with visibility, you must still have 'Imported Categories' (in the visibility graphics box) visible in your view. Any CAD files, even if in imported families can be controlled graphically through this dialogue box.
