Originally Posted by
CommuterRun
Depends on how much you want to see. No mirror can reveal the detail that turning and looking in the desired direction can. It also depends on how much you want to communicate with other road users. I have found that doing a shoulder check seems to tell motorists "watch me; I might be looking because I intend to move over." If I'm viewing them in my mirror they have no way of knowing what I'm looking at.
That is the beauty of using a mirror. It separates the act communication with other drivers from the method of being aware of surroundings which greatly helps reduce miscommunication of intent.