I know two people who have had them. They make the bike extremely unreliable and they broke down a lot. They are hard to get working correctly.
The one guy got it working properly and it was just too much power in the trees. It could outclimb everyone on big climbs because it had more power, but he was the only guy who could make those climbs so he had to come back down and ride what everyone else rode anyways.
In the end with all the hassle, breakdowns and cost both of them sold them and would never do it again.