I have a Cane Creek Thudbuster on my hardtail MTB, and I wouldn't ride without it. It keeps the saddle-to-BB distance fairly constant by arching back-and-down in response to bumps, rather than pogoing up-and-down as most of the other suspension seat posts do. I don't even notice it moving at this stage, except in that I don't get kicked around anymore by root and rocks on the trails I ride. Good for the lower back, too!
The downside to it is greater weight than a standard post, but the advantages it brings in comfort, and improved riding technique (being able to sit more on technical climbs/descents, etc.) is more than worth it.