I bought some of this when OTC clay wouldn't remove some paint transferred to a bumper -- you know, from those steel poles that seem to get in the way when you exit a parking lot.
It was a tad pricey at about $25, and it did still take some effort, but it removed the paint transfer over a pretty broad area of about 4" x 9" -- this was on a '94 Camry (beige metallic). I also used it on the hood to remove stubborn paint splatters that had been on the surface for over 2 years.
I have not noticed marring on the Camry, but the bumper paint wasn't in real good shape anyway; the hood seemed to be have no problems and I followed it with FX SynWax with nice results.
I would imagine on some finishes that there may be some marring with this clay depending on what you use for a lubricant or just how much elbow grease you put into your effort, but I had no complaints.
While I use OTC clays on a regular basis, I like having this aggressive clay handy for those special occassions.