Theo, looking rumpled, post nap
Ok, so I tried the FURminator on Theo, with mixed success. He enjoyed having his chin brushed and tolerated (briefly) some of the longer floof on his haunches, but didn't like his back brushed much. But then he's not a huge fan of being brushed anyway.
I don't think it hurt him, I think the sensation was a bit too intense, as he'd purr and purr but then suddenly the paws would come out to warn me (gently, claws in...) but then he would thrust his face forward for more brushing. I stopped after a short time once his tail started swishing back and forth as this conveys irritation in a cat, he'd had enough.
So for Theo, not sure if it's for him. Ava and Sofia were up for another brush though, and several more handfuls of floof were removed!!! The Furminator doesn't cut or pull the hair, it removes already loose undercoat hairs in much the same way a cat would with its tongue, which is why cats get hairballs, of course. It's amazing how much they do shed though.