Such an important demonstration mission, Dawn is. Not only is it demonstrating the function of ion propulsion en route to asteroids Vesta and Ceres (if you wanna call Ceres an asteroids) - 600 plus days of firing its thrusters and counting!! - but it will also do something no other space vehicle has done yet: enter the orbit of one non-Earth body, then, leave orbit and re-enter the orbit of another non-Earth orbit.
I like the way lead mission scientist put it in an article I read on Space Geographic:
"Dawn isn't exactly a hot rod," says Rayman. "It would take 4 days to go from 0 to 60. But it ultimately achieves fantastically high velocity while consuming very little propellant. It uses only a kilogram of xenon every 4 days."
Maybe a bit hyperbolic, but I also believe it when he says this about the importance of the mission:
"Dawn is taking us all on a virtual trip through the cosmos. It's not just a mission by the JPL team, or by NASA, or by the U.S and its partner countries. It's a mission of humankind -- something that represents all of us who share a spirit of adventure and curiosity, a passion for exploration. It's an extension of ourselves into the universe."