I have never taken a class but going from high 500's to 680 is a big increase. I believe that the way to achieve an increase that large is though mastering material on the GMAT that you are somewhat unfamiliar with at this point. At thins point, I don't think that a class is worth the money.
However, that being said, I hope that your prove me wrong and do exceptionally well. Best of luck. Let us know how you do.
Give Kudos if my post was helpful!