I've used suspended seatposts for three or four years and am a big advocate of them. Most (not all) of the advantages of a true dualie, but they're lighter, cost less, and are easy to upgrade. I've used the U.S.E. post, now called the U.S.E Alien ($139). It combines a spring with a rubber bumper for good bump-absorbing capability and fast return. Heavy or light riders can replace the bumper with one better suited to their riding style and weight. Installation is easy-it's just like replacing a regular seatpost, the only difference being that you'll probably also need a plastic shim that goes inside the seat tube.
Rockshox also makes a good suspended post that sells for only $70, though it lacks the spring of the U.S.E. post and is rubber-only. Still, it's an inexpensive, worthwhile upgrade.