I just read a great article in the Wall Street Journal that pretty much hits it on the head when it comes to the test-prep industry: http://online.wsj.com/article/SB124278685697537839.html. It doesn't specifically mention the GMAT, but it's still an interesting commentary that applies, in some ways, to everything in the test-prep industry. Basically, the article quotes a bunch of academic studies that suggest that the average score improvement from SAT and ACT prep courses is minimal: about 30 points on the SAT, and less than 1 point (!!) on the ACT composite. The article, based on some solid reporting from students at Lowell High School in San Francisco, also accuses test-prep companies of rigging their "diagnostic tests" so that they can claim that their students had huge score improvements.
They wouldn't really do that, would they?
Um, yeah, they would. It's the oldest trick in the test-prep book, and I'm surprised that parents and students are still duped by promises that a certain private tutor or test-prep class "will raise your score by 240 points... guaranteed!" I've been teaching test-prep for nearly a decade now, and I can tell you that there are never any guarantees--some students, for a host of reasons, will never gain 240 points on their SAT score, no matter how brilliant their test-prep course or private tutor may be. (Similarly, there are some people who will never gain 240 points on their GMAT score.) And yes, many companies still rig their "diagnostic" tests so that they can claim that you made a huge score jump.
Don't get me wrong: score increases of 240 points happen on the GMAT, but they're exceedingly rare. Familiarity with the test will definitely improve your score, and a thorough review of basic test content is always productive. A great private GMAT tutor will help you with the psychological aspect of testing, and any good teacher will help you to get a grip on the trickiness inherent in the test. But everybody is different, and there are limits to what prep courses and private GMAT tutoring can achieve for any given individual.
All I'm saying is that it's good to be wary of the claims and statistics. The lively forum debates help keep GMAT test-prep companies honest, but there are still plenty of snake-oil salesmen in our midst.