I don't think a suspension seat post on an MTB that you ride in the woods is going to buy you much. You're better off ensuring that you are off the saddle for rooty/rocky sections anyway, using your legs as your suspension, and the front fork to take up a lot of the slack as well.

Like Hebe, I could never get the suspension post on our tandem just right, and found it pretty uncomfortable. Maybe for the road? But not for real MTB riding.

SheFly