I can't speak for top-of-the-line ones, but the cheap ones SUCK. You're soooooo much better off with a sprung saddle. I hated my suspension seatpost, so I replaced it with a normal one. A couple weeks ago I got a sprung Brooks to replace my unsprung Brooks and WOW it's like riding with a 20 psi tire. The bumps just get soaked right up. I seriously checked my tire pressure after I put it on . It is way more effective and less flimsy-feeling than the seatpost.

If you are looking for a suspension for your butt, I would far sooner recommend a sprung saddle than a suspension seatpost.