A similar drive type, the ion engine, has already been proven pretty fantastic on the Dawn spacecraft.
During this first interplanetary cruise phase, Dawn spent 270 days, or 85% of this phase, using its thrusters. It expended less than 72 kilograms of xenon propellant for a total change in velocity of 1.81 kilometers per second. That's just spectacular. Bring on VASIMR. Its supposed to be mounted on the ISS sometime next year for testing...but if it works well it can actually replace the huge amount of fuel that has to be carted up to the Station to be burned in maintaining its orbit.
Building a spacecraft engine that doesn't need to set anything on fire like some steampunk anachronism gives me all kinds of nerd goosebumps. Chemical rockets basically self destruct when you use them. These plasma and ion drives can be run continuously for months or conceivably years at a time, shut down and restarted with no ill effect, etc etc. Amazing stuff.