Click through and listen to this teacher on using presentations to asses student learning. He lays out perfectly what a valuable use of class time presentations can be if they teacher is working hard to make them valuable. I particularly love that he is going to the groups and telling them ahead of time what his questions will be. This makes the kids feel empowered and motivated to learn.
This tab was to the right of my twitter tab, so I think I owe a debt of gratitude to @fnoschese for this link.