For a controller, look at the Cycle Analyst from Grin Technologies. It is somewhat hackable, can hook up to a crank based torque sensor, or use a cadence sensor. Personally I just use a throttle that "remembers" a given setting when you hold it in a given position for a couple seconds, and throttle enough to bring me up to say 8 mph, then peddle to get to 15 or so. Basically I peddle about how hard I want to, then adjust the throttle to give me the speed I feel like going. But you can also do fully peddle-based (cadence or torque) assist to make it feel more natural (then it would be like riding with a strong wind at your back).