I'm with Retro Grouch, I don't like'em. It's essentially a short travel pogo stick beneath your butt, no way it doesn't affect your pedaling efficiency. They're usually quite heavy, too, compared to a conventional post.
For bikepath cruisers who want a super plush ride, okay, go for it. And in fairness, I've known mountain bikers who swear by them, especially the Cane Creek Thudbuster (uses a parallelogram design). But I knew a guy who spent a small fortune building a very light hardtail race bike once, and then he put a Thudbuster on it. Seemed ridiculous to me, but to each their own-