I could not resist responding when I saw Veritas in the post...
Let me start with the larger point and the title of your post - "should you take a prep course?" You might think that I would give you an automatic "yes" but there are actually several things to consider:
1) How much improvement do you need over your current level in order to be competitive at the schools to which you will apply? You should take one of the official GMAT practice tests, called GMAT Prep - if you have not already done so. You should investigate the mean GMAT scores for the schools you would like to attend. And you should think about the other components of your application. (If you have less than five years work experience you will want your score to exceed the mean GMAT score for that school). Once you have come up with a target score see how close you are currently to that score. If you are very close - say less than 50 points away, you might not need a course. If you are 100 points away or more, you should definitely consider a course.
2) How many months do you have to prepare and how much time per week? If you do not take a course you will be "re-inventing the wheel" so to speak. Even using study guides there will still be things you do not understand and will have to figure out for yourself...things that an instructor has probably dealt with many times with many students. If you have any limitations in your schedule realize that a course is more efficient than figuring it out on your own. If you have lots of lead time before the test and like to do things on your own then maybe you can go that route.
3) Are you the kind of person that reads the rules to a game or contest and immediately starts thinking of the best way to win? That may sound like a strange question but we all have strengths and weaknesses. A strength for most GMAT instructors is the ability to find the most efficient way to do something using a given set of rules. I have lots of stories of success in very different endeavors all based on my strategic thinking. That is what makes teaching the GMAT so much fun for me. Some people are better if they have more structure, lists and rules and step-by-step processes, that is the type of thing a course provides. We do much of the imagining and strategic thinking for you.
As to your second question, we do happen to believe that Veritas is the best value for several reasons, the quality of our instructors, every one of whom has scored in the 99th percentile on an officially administered GMAT and must complete more than 100 hours of training and serve as apprentice instructor before getting their first class; the number of in class hours - 42 hours over 14 sessions for the full course more than any other leading provider; the quality of our materials; and the fact that we a no questions asked guarantee. Most important, however is the success of our students, more than 1/3 of our students score above a 700 on the GMAT! To see more just click on Veritas anywhere in this post.
Hope that helps! Good luck whatever decide and keep us posted....