I teach for one of the larger course providers; I won't mention which one.The answer to your question really depends on several different factors.(1) Where are you scoring right now? If you're near the 50th percentile, it's going to be very difficult for you to see much improvement. The bell curve at that point is so shallow that you need to get MUCH better (or worse) to end up with a different percentile score. Likewise, if you're scoring below the 10th percentile or above the 90th percentile, the likelihood of realizing a significant gain is slim. Those who are most likely to benefit from test prep courses are those who are scoring at around the steepest points on the bell curve--between the 25th and 40th percentiles, and between the 60th and 75th percentiles.(2) How hard are you going to work? Taking a prep course, in and of itself, isn't going to do you any good unless you invest your own time and effort into learning what you are being taught. That means doing ALL of the assigned homework and taking ALL of the practice tests, at a bare minimum--courses also offer extra materials beyond those which are required, and they can help you too. The bottom line: enrolling in a course is not a quick fix.(3) How smart are you? Some people are just plain stupid--we all know this but rarely say it. If you're one of those people, don't waste your time or money. For example, chances are pretty good that someone scoring in the 10th percentile is going to be too dumb to understand what's going on in the course. If, however, you're reasonably intelligent, you're going to get more of a benefit. Those who tend to benefit the most are, in my experience, students who are exceptionally intelligent and hardworking, but whose scores are low because of time-management problems, because of test anxiety, because of poor strategy, or because they need an intensive review of the material covered by tests that measure familiarity with content (such as the two science sections of the MCAT, or the GRE subject tests).Good luck!!!