Ion thrusters aren't comparable to common chemical engines and they don't need to be. They can't offer same high acceleration (to enable escaping the planet gravitation for instance). What they can offer however is very long operation and constant acceleration (even if not high). What it means that when in outer space you can accelerate the spacecraft to huge speeds over long time, which you can't do with regular fuel, simply because you quickly run out of it. It's not a revolutionary idea by any means (at least not anymore) - it is pretty well known.