Mountain Bike Action: First, we don't have an adjustable-height seatposts at the top of any list. The products we have tested so far have not proven reliable enough for us to recommend them to riders without a warning. The RockShox Reverb shows the most promise (we are testing them now) and the Fox version that has been in development for years just might make it to bikes for 2013. All the adjustable-height seatposts available today will probably need to be rebuilt within the first year of use. In a way, riders using these seatposts are unpaid R&D riders.
To answer your question, there is not a ride-quality difference between fixed and infinite. You won't feel the difference between an inch lower and say, an inch and a quarter lower. Most adjustable posts drop three inches and this seems to be an ideal drop from your normal saddle height in order to really feel and benefit from the lower center of gravity. Points between the two extremes are fine, but we find riding with it topped-out or bottomed-out works the best.
By the way, riders who switch to an adjustable-height seatpost don't go back to a conventional post even with the weight penalty (about a pound) and reliability issues that come along with them. It makes a big difference in riding performance.
The yet-to-be-released Fox adjustable seatpost has us sitting on the edge of our saddles in anticipation.