One thing I'd do is ask about your situation in the admissions consulting thread. You don't mention where you want to apply, but depending on that and other things such as your work experience, GPA, etc, your time may be better spent on applications. 680 is a great score. I know it's not what you wanted, but it's still a high score.
You don't mention whether you took our course but, if you did, contact
studentservices@manhattangmat.com and ask for a Post Exam Assessment. This is a free service for our students after they take the official test - you can debrief with an instructor and set up a game plan for taking the test again.
Whenever this test gives you a problem, the issue is not whether you can answer it. The issue is whether you can
answer it in 2 minutes. If you can't do that, you HAVE to let it go, because it costs you a lot more at the end. 3-4 questions in a row wrong at the end results in probably a 4 to 8 percentile point drop right there. It's a lot worse to get questions wrong in a row, and it's a lot worse to get lower-level questions wrong than higher-level questions. By definition, a question on which you need to spend 3-4 minutes is a higher-level question for you. And when you take any 3 or 4 questions in a row, at least one of those questions is "below your level" - so getting it wrong will really hurt you. (And probably more than one of those questions, because remember that as you keep getting problems wrong, the subsequent problems are getting easier.)
You can still take both GMATPrep and MGMAT CATs as long as you follow a few guidelines to minimize the chance of artificially inflating your score via question repeats. First, anytime you see a problem that you remember (and this means: I know the answer or I'm pretty sure I remember the answer, not just "hmm, this looks familiar..."), immediately look at the timer and make yourself sit there for the full length of time for that question type. This way, you don't artificially give yourself more time than you should have. Second, think about whether you got this problem right the last time. If you did, get it right again this time. If you didn't, get it wrong again. If you *completely honestly* think that you would get it right this time around if it were a new question (even though you got it wrong last time) because you've studied that area and improved, then get it right this time.
Most of the time, people tell me that they see only a few repeats on GMATPrep. The same goes for at least the first couple of our tests. After that, you may start to see more repeats. But if you follow the guidelines above, you'll minimize the impact that the repeat has (in terms of skewing your results).
Re: OG, you're not done with those. If you still want to improve, then you haven't learned everything you could've learned from them and the real thing is the best source from which to study. That doesn't mean you shouldn't also use other stuff. You might also try GMAT Focus for quant (unfortunately there isn't a verbal version). I have had some students say that LSAT material is good practice for the GMAT but I have not tested that theory myself and it's been more than 10 years since I last taught the LSAT.
For CR (and verbal in general), it's worth it to spend time analyzing the wrong answers. Be able to articulate:
- specifically why each wrong answer is wrong,
- which wrong answer is the most tempting and why
- how to recognize that the tempting wrong answer is still wrong anyway so you can eliminate it
- why someone might be tempted to eliminate the right answer
I don't generally recommend serious study of the math challenge problems unless someone's already scoring in the 95th+ percentile on math and just wants to have some fun. Most people don't find these fun. :) The vast majority are harder than anything you'd actually see on the test and some could actually have a detrimental effect, in that you could train yourself to do things in a way that might be best for the challenge problems but would not be beneficial for real GMAT questions.