I'd like to just elaborate on how the ion engine works. A ion engine/electric rocket uses essentially a radio antenna to strip the fuel's atom's of their electrons. These newly formed cations are expelled out of the engine at high velocity (~290 000m/s, VASMIR engine) using a magnetic nozzle. Unfourtunately because the mass of the cations is so small it does not accelerate very fast. As everyone has said, it has very low thrust but very high specific impulse. It can go extremely fast but needs the time to get there.
There is one other problem with ion engines. The speed that they can travel at diminishes over time because all those stripped electrons have to go somewhere so they are stored on the space craft as there is no ground to ground the engine. It takes a lot of voltage to keep all those electrons in check. Seen as the power supply is limited in voltage on the space craft eventually the voltage used to accelerate the ions goes down and the speed of the exhaust is less. So if that happens before the maximum speed is achieved the engine travels stays stuck at that speed even though it still can have fuel. One of the first uses of an ion engine was for the Deep Space 1 mission (Google me!).
I hope that helps.
Mike
Oh btw, I'm new on this forum and I cannot find the introductions section. The only one I can see that I could post an introduction is in the general section.