I prefer the controlled adjustability of two bolt seatposts -- I don't think anyone has mentioned that so far. With a two bolt system, you can loosen one bolt, slide the saddle fore or aft, and tighten that one bolt: tilt stays the same. Or you can loosen one bolt and tighten the other to adjust the tilt -- fore/aft position doesn't change. Adjustments are precise.
Contrast that to my experience with one bolt systems. Loosen the bolt until things can move. When you get the bolt loose enough, everything is in play. I've gotten the tilt right only to mess up fore/aft, and vice versa. Adjustment is more of an accuracy thing; it's like learning to shoot a bow and arrow. Miss right? Go back and shoot again, it may be left, up, or down from the bulls-eye. Keep shooting (or adjusting) until you get lucky and hit the bulls-eye, or get tired and call it quits.
Drew's picture surprised me. I haven't had a failure like that on any of my two post seatposts. I guess that was a latent manufacturing defect -- nasty surprise when it failed, I'm sure!