Just thought I would share my experience with the Ezip, as it seems to be the most popular ebike around, probably due to its low price.
I bought it because I found it at my local Toys R Us for $300 on clearance for the 2009 model. I'm a moderately experienced mountain and road biker, but I've just recently started communting to work, so I thought an electric would be in my future to get me there a little quicker with less sweat.
I don't know what I was expecting for $300.... You get what you pay for. The Ezip is a very poor quality bike. It's a walmart bike with a motor on it. And as an experienced biker, I always tell people to spend a little money on a bike. If you buy a Walmart bike (any bike under, say, $400) you won't like it and therefore won't like biking. Besides weighing 75 lbs, (any good non-ebike will weigh a third of that or less) it has the worst components you can find that work very poorly when brand new and will last a very short time (I know, I've seen family members go through tons of these kinds of bikes). The motor, charging system and battery seem fine, although the battery is quite big and heavy. I know that some, more powerful batteries weigh half as much.
So I got on the road and clunked it up to speed. This bike is not fun to use. It feels like you're hauling around a bathtub. The motor engages well and pulls the bike along fine, I guess. Top speed, at around 15 mph is slower than it seems on paper. I realized that I bike much faster than this on my other bikes just pedalling slowly and moderately. With a good amount of extra pedal power you can coerse the bike into getting a bit faster, but you're limited by the top gear of the bike. To get top speed on this bike WITH the electric assist going, I figured I needed to work twice as hard as on my other non electric commuter bike.
So after a long test run, I figured, "what's the point?" I'm not one of those road bikers that train everyday, I don't own any spandex, or anything like that. I think I'm in reasonably good shape but not top of my game good or anything. For someone like me that knows what a good bike feels like and isn't very out of shape or overweight, there's no reason to own a bike like this. It sucks the fun out of the whole experience.
I can see how people who are VERY out of shape, or overweight (although keep in mind, the posted weight limit is 240 lbs) might like the added elecrtric assist, and they might even find it gets them in shape seeing as you really do need to give a good amount of effort if you want to save the batteries or go faster than a snails pace.
But if you don't fall into that category and you're considering one of these, please please please go to a bike shop and test ride a good bike instead. They'll go faster with less effort and just feel so much better. They'll be fun and agile and will last a lifetime if maintained. Plus you'll get the exercise and feel good about yourself for using pure human power. I paid $300 for my Trek Soho single speed in mint condition off of my local craigslist. I would take that ANY day over an Ezip.
I was curious about electric bikes, but I see now there's a reason why people still ride without batteries.