I don't really see it as much different than the old pie-in-the-face, and it probably hurts about as much as high-diving face-first into water. Then again, you're right: get the wrong person and you've got a potential heart attack.
This is extremely phoned in for a Japanese prank. They were really having a "what the fuck ever" day with this one. Three stars, plus one more for those glasses.
Living in Japan would make me paranoid as fuck that I'd end up being the victim of one of these pranks.
Japan must have more strict limitations on filing civil suits than we do in the west because I've seen so many Japanese pranks pulled on shows like these and I can't believe no one sued the shit out of the people responsible. The one where the chair inside the spa at the ski resort throws the poor naked bastard out into the snow in front of all the skiers comes to mind.