@akuma: Of course, the 30-point interval can also work against you - it's not something I'm disagreeing with. It's just that once you're at 680, a 700 is more accessible to you than to a person who scored a 500, for instance.
@misterchipper: As to which plan you should be following, there are a number of options. Did you go through the MGMAT guides thoroughly or do you feel that maybe you missed some stuff on your first go? If it's the second option, then you might gain the 20 points you're missing by simply redoing the books. If it's the first one, then it's a bit more complicated. For the two math topics that you've mentioned, I have not reviewed any resource that's better than MGMAT (they are pretty advanced topics). For CR, maybe you could take a look at the PowerScore CR Bible, a book I personally loved. It's also good for RC!
You say you're thinking about getting into another prep course... The answer to that depends solely on you and your study habits: do you study in a structured manner on your own or do you need some deadlines to keep you motivated? Is it intimidating to think that your questions cannot be answered live by a teacher (remember, you can always post them here on the forum for some good, debated answers)? Do you feel that you could gain a lot by going through the basics once more (because you will do that, since the course is designed for people who start fresh)?There's also a plus side to enrolling in the Knewton course: since your baseline score will be a 680, unless you get a 730, you won't pay a dime for the course. But again, it depends on your way of studying!
Then again, math is math no matter how you put it and no GMAT book will be an easy read.
