Anyone use them? Do they have to be specific to a given make of bike or can they be generic?

it makes a huge difference on brand and setup. like on a birch the bike has to be setup for it and the motor with the right software updates. same with the other big guys. either bikes who knows? most bikes are nto setup for them though it seems it's usually cheaper to carry another battery.

I have an Orbea Gain. I purchased a range extender for it in the spring. The extender is made by Orbea and designed for this bike. The only downside to it's design is that it takes the place of the seat tube water bottle cage. I got it to help my mileage per charge in the winter. But found it especially useful in warmer weather also.