"The Gen1 Enterprise will be powered by three ion propulsion engines. These will provide constant acceleration, and versions of this technology are already used in spacecrafts."

to my knowledge they are used in spacecraft that can afford acceleration cycles that last several years and "rapid" maneuvers that last several months. ion engines produce very little thrust but for a very long time. i'm not sure how practical they would be for anything except a lifetime spent aboard a ship on a never-ending quest to reach somewhere distant.