The good thing about GMAT prep courses is that they force you to adhere to a set schedule which may help you stay disciplined with your studies. You also get to interact with other students in class and share ideas. But, you need a great GMAT private tutor online to learn better. However, prep courses aren't tailored to your unique needs. The typical class is geared towards an average student looking for a mid-500 level score.
If you're hoping to do better then you may find the instruction lacking. Also since the course must appeal to a broad audience the classes will cover every math and verbal topic on the GMAT. If you are strong in one of these areas then you will be wasting some time going over things you already know rather than using that time to work on the areas where you aren't so good at.
The schedule might not always be convenient for you either. Things come up all the time that can get in the way. But if you miss a class or can't do the homework for a week then you will fall behind in the larger class format. Another common complaint as well is that the quality of the teachers can vary greatly among the larger preps.
The main question is whether they will give you special tricks and strategies. Be skeptical when reading the advertising from larger prep companies. What you cover in the classroom might not add anything new so the extra cost for the in class work might not be worth it. So, the need of GMAT one-on-one tutoring is always helpful and much needed for the students.
Various educational groups have criticized these larger test prep courses and pointed out that there is little in the way of actual solid evidence to back up their claims of being able to get large increases in scores. For example, mock GMAT tests are often constructed so as to show scores that inflate once students take the actual GMAT. Although a prep course could conceivably increase your score by as much as 100 points, this is not common. The thing to remember here is that while the study and assistance you get will certainly help but you should always question the marketing material and promises you see.