After launching the duct command, then selecting a connection point, I find it very useful to be able to hit the space bar and have the new duct match the elevation and size of that connection, however I wish that it would also match the shape.
I draw a lot of round & rectangular ducts on the same plan, and constantly having to change the duct type before actually drawing the duct becomes a time consumer. Matching the duct shape as well would streamline the process of drawing ducts.