Great for those that benefit from them. Personally, I'll stick to conventional propulsion, until some point my back really gives up. They do open up opportunities for those with health conditions. Not so sure of the benefit to healthy folks. One of the lads I go MTB'ing sold his as he got fed up of waiting for everyone at the top of hills - he was fit and healthy though. One fella turned up on a ride on one, and it did make climbing look easy, and he was the only one who got up one particular hill. That was the only time he was any quicker.
One thing I would say, is most of the folk I've spoken too have had reliability issues with the motors on MTB's due to the absolute filth we ride through. I still think there is work to do on motors and batteries though, and the crazy battery prices need to come down.