I went to SpeedRiver Bicycles here in my town and they sell the BionX systems that they can install on your choice of bicycle. The BionX system has a lot of features that make it very desirable over other systems I've seen searching on the web. The system is stronger than I expected according to what the guy I talked too said. He was very knowledgeable about the product and the company itself. He actually had a guided tour of their facilities. The system wouldn't have any problems hauling a big 225 pound guy like me around tackling steep hills. I like the regenerative feature of the system.
http://www.bionx.ca/en/products/technology/