It still puzzles me to a great extent how people can become so excited when they hear about the ion engine. Yes, it is a smart design. Yes, it is very energy efficient. Yes, it can propel spacecraft to very high velocities. But NO, it cannot provide a high accelaration: it took months for the probe to reach the Moon, and, adding insult to injury, used a gravity-assisted flyby of Earth!
Then there is a different problem: how do you brake a probe equipped with only an ion engine? Right now they used the Moon's own gravitational field to capture the probe. Gravitational fields of sufficient strength are rare :-).
It is a promising technology and deserves lots of further study, yes. But until an ion engine can deliver the same thrust that will propel us to the Moon in 4 days, and have the stamina to keep that same level of thrust up for years, then and only then will I begin to call it 'Star Trek' technology.