I haven't tried one yet, but in Japan, they are really getting popular.
The Japanese are pretty sharp. If electric bikes are popular in Japan, then electric bikes are probably pretty functional.
Here in the States, bicycling is primarily recreational. Obviously, electric bikes don't offer much in terms of recreation - especially when there are high powered gas engine alternatives for recreation.
It will be interesting to see how they fair in other countries where bicycles are used more practically. I have seen them in China as well, but not really prominant yet.