For my money I'd rather a proper integrated system like the Panasonic range: http://cycle.panasonic.jp/products/electric/

I've ridden a number of electric-assist bicycles and the panasonic system worked brilliantly - seamless and intuitive boosting exactly when needed, so hills just seem to disappear. They seem a lot more convenient, too, with the battery integrated into the lights system.

Yamaha's pedelec system is also worth a look, though I didn't find it quite as intuitive as the panasonic: http://www.yamaha-motor.co.jp/pas/lineup/