I've ridden the Shimano, Bosch, Yamaha and BaFang mid-drives. I liked the Shimano very much, but it is NOT rated for off-road use like the Bosch and Yamaha. So if you want to MTB get something else or go with a BaFang mounted on a standard MTB (I own one and enjoy it very much).

The BaFang BBS## are GREAT RELIABLE SMOOTH motors and the most reasonably priced solution for the majority of people. Don't let anyone tell you differently! They are the GO TO bolt on solution and give you the option of using a throttle or boost button for extra power.

If you are only going to ride designated bike trails or commuting on the street, take a look at the Raliegh Misceo Shimano mid-drive system with electric shifting and automatic gear reset when you stop.