I've used it.... depends on the student. Very good for someone who is all arms to 'feel' what connection is all about, and pivot.
But I would only use it briefly to get the point across, and only for chip and pitch shots. WAY too limiting. Something to borrow, not to buy.
Brian - I disagree a little about extensor action. It IS actually quite good at letting someone feel what that is, the problem is that it only lets you 'start' to feel it - agreed that you can't extend properly - but it does put you in 'position' to 'feel' the begining of extensor action.... puts you on the right road, but doesn't let you drive down it (literally).