I actually think that a suspension seat post can be a wonderful thing. If you are riding over very very rocky trails, it can allow you stay seated and thus will improve your bike handling. That is exactly what happened to me when I lived in phoenix and regularly rode South Mountains trails on my cannondale F1000 hard tail mtb. The rock shox seatpost greatly improved my riding there. And I am not talking about wussy terrain either, I was able to climb all of National Trail (except the 30 foot waterfall area) after getting that post.

I did not notice a definite change in leg length because I was so busy noticing that my seat was no longer being shoved up my rear end by anything larger than 2 inches.

For road bikes, I am not sure.