4k cameras (4000 frames per second instead of the usual 70-something) are used to slow things r i i i i g h t d o o w n. They're quite expensive though, so you tend only to see them used to publicise rugby matches on the BBC (so THAT's what the license fee is for).
Sky One's Brainiac, however, does exactly what I would do with them. Hits things (and people) and films the reaction, v e e e e r y s l o o o o w l y. Watch this video of a chubby man getting slapped in the face. It's quite funny (interesting too, to note how flexible the dude's nose is).