Vader will basically provide you directions, empire-style, and will even congratulate you with an “impressive” when you get to your destination. The voices are randomized, so that you won’t always get the same sentence for a given situation.
$12.99 is a bit stiff, but I guess that Star War fans are known to not spare any expenses to live their dreams. What do you think? Oh — and if you don’t feel like joining the Dark Side, Yoda’s voice is also available. [Vader, Yoda]