I have a Merida 29er, about 13 years old. My knees hurt so I decided to go with the RevBikes 250W rear hub torque sense motor with the mini battery because I wanted it to look inconspicuous. It took me a couple of hours to install because I was being careful, I reckon if I did it again I could do it in 30 minutes. Everything just fitted in place. The derailleur didn't need adjustment, and neither did the rear brake. I fitted the controller so it laid back over the headset so it would in inconspicuous. I need the Triple Bob spacer to move the battery up about 10mm away from the downtube. This worked fine, so it's good to check the installation video, which highlights the correct distance from the down tube to the first bottle holder bolt (215mm, mine was 195mm). The wiring to the battery and controller were plug and play. There's two unused black and white wires out of the battery controller and three unused juliet connecters out of the controller. I guess something to do with brake cutouts, so I just taped them out of the way.
The YL80C controller is basic but works well. It's hard to find the manual for this particular Australian unit. There is no sub-menu, just a bunch of P01 to P20 screens. Not much worth changing except the maxspeed. I delved deeper and found that the trip meter can be reset without affecting the odometer by selecting Trip and then holding the Info and Minus buttons for a couple of seconds.
I found it had plenty of power in setting 1. I don't feel the need for brake cutouts, and there is no drag when coasting. I greatly prefer the torque sensing drive over the pedal sensing bikes I have hired. It feels much more natural.
Highly recommended, thank you.