680 is a fairly good score but it all depends on the schools to which you are applying. If you want to re-take the exam, consider this:
rom one test to the next you can easily jump +/- 30 points (and even up to 80 or so) depending on where the mistakes are. So you can get the same percentage of right and wrong on two exams with the same amount of knowledge and yet have a score that differs by quite a bit. This is the same as the real GMAT. To illustrate, I can make a mistake every other question in the Quant over 5 exams (ie in every exam my percentage is 50% but have a score range from, say, a 15 to a 35, for example, which is quite insane, but it shows too that where you make mistakes can often be more important than how many. Overall, when a student wants to know where he/she stands, the best way to do so is to take the average after a few exams. This should get you to a fairly close estimation of where you are at. You can also check out our blog for more info on GMAT scoring (gmat.economist
I do wish to add the following. While I like to encourage students to retake exams if they have the energy and time to do so, in certain circumstances in may not be necessary especially in the context of other application criteria. So altogether a retake is good for students who are both far and close to their GMAT range but this will take dedication and further practice. If you have it in you to do so then go for it! If you have other aspects you need to take care of but are already at your range then I do not see it as all important. The other aspects of the application may carry quite a bit more weight. And finally, you can always ask the school itself whether it is worth doing. Sometimes they actually help in that regard.
Best of luck,
Economist GMAT Tutor