I would expect to see Lazarus jump into this thread. He just went through the same series of questions you asked, and has started using his newly converted e-bike.
Obviously it's a great way to get around if you're concerned about efficiency but want to be able to use 2 wheels outside of your normal comfort zone (distance or speed wise).
I know you don't need convincing, but did you see this thread?
Test: 250w electric bicycle efficiency = 1512 MPG equivalent