Once upon a time I was into this sort of thing. My experiment was with an underpowered electric scooter from Toys-R-Us, eh the error. But when I started cycling to work on a daily basis, I simply got my bike legs.
Glow Worm Bikes, around the corner from Reverse Garbage sells these types of power assisted bikes and kits (I suspect but did not look too much into it). And they are not shy about their asking prices -- thousands of dollars. So the value order goes something like: electric bike, modest used car, regular bike, my single speed.
I would like to witness first hand the electrics in China. It is said that they are everywhere. There must be a lower barrier to entry. One would further imagine that the Chinese would rather upgrade to cars rather than electric mopeds if the costs are similar.
The thing with Aussie is that we have a 200W motor restriction (as used in larger children's scooters), so well made electric mopeds are effectively forbidden by the grand unified wisdom of our government, hence the view on the ground is different when in comes to these types of vehicles. That is not to say that I have not seen some cool hardware in Sydney, but that's another matter.
Here is 2 dudes that I snapped in NY during my travels. These are quite different in therms of speed, range and utility compared to the example above.
I wish I could remember where I found this tandem electric, but I figure that it was around Marrickville because it looks like a Glow Worm.
And then there are these rare mid mounted motors. I found this example one on Illawarra Road in front of Banana Joes.