I have a Stokemonkey, which drives a left-hand crank through the BB. I climb several short but steep grades (max of roughly 20% grade based on mapmyride) everyday. I have ridden DD hub kits, but not on this route. Here are my observations so far:
1) Not withstanding the inefficiency of gearing up and down, the system works well. The motor spins where it is happy pretty much all the time. It will do the 20% motor-only with very little current, but as many have observed, it will not do it fast. To go up fast, you need to think about power. To go up slow, you need to think about torque. Or at least, that is the way I think about it. At low bike speeds, nothing is moving very fast and so whatever inefficiency there is is not hurting much. The gains you get working through the mechanical advantage of the gears will far outweigh any disadvantages.
2) Putting extra torque through the cranks is only a problem if you try to do much more than you could without the motor. I think of the motor as a way to do what I COULD do solo, but which would leave me gasping for breath with burning thighs. Ride the bike as you would without the motor. That is, never shift under load. You are begging to bend a deraileur if you do. This is doubly true when powering the crank. However, I've put 2800 miles on my system now and there is no apparent effect on the rear or front deraileur. I always de-throttle to shift, same as I would pedaling.
3) There are more moving parts and presumably more things to go wrong. However, at least in the SM system, the additional parts are relatively few and do not rely on complicated mechanisms. The SM system has one extra freewheel (on the motor) but none on the crank. You are not likely to wear out the short additional non-drive side chain or the extra ring gear. The freewheel on the motor is not under much load and I can't tell any effect on mine so far.
4) So far, the only problem I've had is a BB bearing failure. But this bike had a cheap BB and I suspect there were problems from the beginning (could hear knocking in BB even before adding the SM).
5) I suspect a DD system run conservatively will have fewer maintenance issues than mine. Then again, I often carry an extra 50 lbs of groceries, etc. on mine. I'm pretty happy so far. I ride to work 90% of the days, rain or shine, cold or hot. I am liberal with cleaning and oiling the chain. So far so good.
6) While I suspect the owner of SM might beg to differ, I think its strength is hill-climbing and moderate speeds. If I were a speed demon I would go for a hub motor. But for flat ground speeds around 20-25 mph mixed with some steep hills, the SM is pretty hard to beat, and that describes my commute to a tea.
I just shelled out for a DD 9C 6 x 10 to experiment with and when I have some experience with that system on this route, I will compare them. I suspect Dogman's observations are correct; that is to say, a little pedaling goes a long way with a DD.
