Hi dont_give_up,
First off, a 680/Q49 is a fantastic score (it's right around the 85th percentile overall), so it could be enough to get you into your first-choice School. As such, a retest might not be necessary. Depending on the Schools that you plan to apply to, you would likely find it beneficial to speak with an Admissions Expert about your overall profile. There's a Forum full of them here:
ask-admission-consultants-124/GMAC has publicly stated that the Official Score that you earn on Test Day is within +/- 30 points of actual ability. Assuming a similar 'swing' in how your CATs function, your results show a general ability level right around 710 (with a high of 740 and a low of 680). Before you rush in to retest, you really have to consider all of the details around how you took your practice CATs (and taking the CATs at home is NOT realistic at all). You might just have had a 'bad day' on Test Day, but here are some other things to consider about how you took your CATs:
1) When did you take your last practice CAT?
2) Did you take them at the same time of day as your Official GMAT?
3) Did you ever do ANYTHING during your CATs that you couldn't do on Test Day (pause the CAT, skip sections, take longer breaks, etc.)?
4) Did you ever take a CAT more than once? Had you seen any of the questions BEFORE?
GMAT assassins aren't born, they're made,
Rich