This is developing into a very interesting thread. I've already learned a few things particularly with regard to the evolution of Pro/E's tools for dealing with scan data.
Raindrop GeoMagic definitely is not parametric. I will be experimenting with REX next week, but I may have incorporated some parametric functionailty into the process. Raindrop GeoMagic can provide dimensional information in the form of NURBS surfaces that import as dumb solids (no parametrics). Raindrop GeoMagic is in the process of adding more basic geometric constructs (planar surfaces, cylinders, extrusions, etc.), but it was originally developed to accomodate free form scan data and wasn't developed with CAD functionality in mind. PTC is apparently trying to bridge the gap between free form scan data and CAD, but I can't speak to the effectiveness of their tools until I learn more about them.
I'm looking forward to seeing what REX can do. My initial exposure made it look promising, but I suspect it has a some limitations at this point in time. Integrating a free form modeler with a mechanical CAD tool sufficient for detailed design and engineering is a daunting task. I have been looking for such a tool for several years now and everything I've seen has serious limitations. The reverse engineering work I perform includes alot of traditional engineering that software hasn't been able to provide.