It's interesting that most of the responses here point out that an e-bike isn't really a substitute for a bicycle, but is a substitute for a car. (fast commute with no sweating, hauls cargo easily, etc.).
If you view a bicycle as an exercise machine then an e-bike isn't a good substitute for a bicycle, except if you're out of shape.
If you view a bicycle as a toy, well, hey, an e-bike is a lot of fun for some people.
If you view bicycles and cars as useful transportation, e-bikes have some of the advantages and disadvantages of bicycles, and some of the advantages and disadvantages of cars.