First time I've seen this discussion, but agree with turbo and cycleops that factory systems appear superior to bolt on units at least for ascending tortuous terrain. My BBS02, with only an rpm sensor, climbs erratically (FOR ME) on a tough, craggy hill and I need to keep my cadence smooth, sometimes difficult with a big bump or switchback in the trail, or the power shuts off. The same area on a Bosch-equipped system is much easier since the unit detects torque (as well as bike speed) and will continue to provide assistance. Also, qualitative results suggest the factory system is more efficient although I haven't done anything definitive.