Try cutting steel with one, then you know what scary is...
I'm not man enough. I also don't know how to quote 2 posts so:
"Try cutting through 1.5" thick computer flooring with one.....takes FOREVER, and ruins the cutter pretty fast."
Carbide tipped holesaw, the SVT (or HiWatt) of holesaws. I've used 4" and 6" diameter ones to cut thousands of circles out of 3/4" MDF.