I paid $15 (plus shipping) for some off of Amazon. They're wonderfully simple and quick, and easily hide-able, which is nice. Everybody's already mentioned the good and bad points. Like Nick014 said, they can work really well in tandem with a spreader bar. I was able to totally secure my subs feet for tickling fun that way last time we played. It was so much fun!
Later in the same session, she popped one of the cuffs free when the cheap link gave. It was a quick fix, it does point up that some sets are not quality crafted (mine came all the way from China).