I've been following these for several years now. For a variety of reasons, particularly the ruggedness of the terrain where I live, and my concern over my slightly-damaged knees.
I finally took the plunge last week, and took delivery yesterday of my first hybrid vehicle, a Trek SuperCommuter +8S bicycle. It has the snazziest Bosch power-assist technology, nicely-integrated into the bicycle.
You still have to do work, the battery assist system merely provides boost to your input, with 4 levels ranging up to 300% increase. The battery has sufficient range for ~90 miles of riding if you don't leave it in Turbo power mode and ride straight up mountains constantly. In that use-case, which is mine, you get about 30 miles or so.
The assist system only assists up to 28mph, after that you are on your own. On very steep hills, it will not be going 28mph unless you have legs like Conan, but you can still manage a good pace.
It recharges in ~4 hours. The charger isn't terribly heavy, but is about half the size of the battery itself, so it takes up a little room in your day pack if your intent is to top up the battery at your destination.
The ride from town to the entrance to my community's road system, ~4 miles, takes me 13 minutes on this bicycle, up a mighty steep hill. It's 8 mins by car (low speed limits here). There's about 600 foot of elevation gain, most of which is the last mile or so.
On the flats, I can easily and happily pedal at the 28mph. I can't go much faster due to the gearing of the bicycle, which tops out at about 30mph. I've broken 50mph downhill, but that's cheating.
Using it with the power system off is possible, the bike weighs 52 pounds however, so it's a bit of a pig at that point, not too bad though.
This bicycle really takes the sting off older/damaged knees. It also removes a lot of the pain of the winds we get here, and flattens out the moderate hills so you don't even notice them. You still have to work, especially on the severe hill, but it's not an unreasonable amount of effort. Even going up the nastiest bits, my heart rate never exceeded 125bpm (for reference, when I'm doing heavy exercise for conditioning purposes I target 140-160bpm, my resting heart rate is 55, and I'm 56 and way too heavy)
I think this technology is a gamechanger for older or disabled or less-fit individuals, and for people who live in places where the terrain challenges bicycles.
I rode to my friend's house for lunch today, 20 mins to her door from mine. Takes me 17 minutes by car on the same route. Took me 28 minutes coming home, because of the mountain issue. I didn't even arrive all smelly/nasty from biking.
Go technology, and happy Earth Day.