If they succeed on large scale tests, this will be a quantum leap in space exploration - opening the door to greatly speed up, and increase the range of, exploration of our solar system, including places that may harbor some form of life.
Plasma rockets (or electric rockets) are less expensive to launch than conventional chemical rockets. They need to use much less fuel to push the same weight as conventional rocket engines, and have longer engine life. In addition, the costs for human exploration are less as well: shorter trips mean less supplies are required. Another important bonus for human explorers is that a much shorter trip means they will receive far less cosmic radiation, and will have less bone loss.
"VASIMR page." Ad Astra Rocket Company. Ad Astra Rocket Company. n.d. Web. 16 October 2009. <http://www.adastrarocket.com/VASIMR.html>
Zyga, Lisa. "Plasma Rocket Could Travel to Mars in 39 Days." PhysOrg.com. PhysOrg.com. 6 October 2009. Web. 16 October 2009. <http://www.physorg.com/news174031552.html>
Image credits: Ad Astra Rocket Company