yes, you can still change gears. The easiest to install is the front hub since you don't have to mess with the back gears at all. If you install a front hub, you need either a chromoly or steel bike because it has to be strong to handle the motor. I think you can use an aluminum bike if you install a rear hub motor but I'm not sure about that.

With a direct drive, there is some resistance but I'm still able to pedal mine. I can pedal my forsen hub motor at 18 mph with the motor turned off. I would say the difference between pedaling a regular bike without a motor at all and pedaling a direct drive is like the difference between pedaling on pavement and pedaling in the grass.