Without knowing the rest of your profile (Undergrad GPA, work experience, etc), the obvious answer is yes, especially if you were doing well on all your practice exams. Maybe you had a bad day?

660 is closer to the bottom of the 80% range at the top schools. It's not impossible, but knowing that roughly 10% of the incoming class got admitted with a score of 660 and below doesn't exactly give anyone a warm fuzzy. If you're confident you can do better, I'd schedule another sitting ASAP so your test prep doesn't detiorate, and so your GMAT prep doesn't conflict with your applications (assuming you're preparing for round 1).

Best of luck!

Without knowing most of your details, it seems as though you should retake the exam. You really don't want your weakness to be the GMAT if you have the ability to do well. This is one of the few things you can change when applying to schools.