There's a world of difference between low-end systems which simply detect that you're pedalling and switch on at some set power level, and high-end systems which sense the amount of torque you're producing and add assistance proportionally. Understandably there's a fair bit of difference between the various manufacturers' implementations, too, which can result in very different feel.
It's cheating, but sometimes cheating is beautiful or even appropriate. I must admit I'm curious as to what one of these mountain bikes is like to ride off-road...