First and foremost, you need to make a couple of self assessments.
-Are you the type of person who can motivate yourself to study in a disciplined manner.
-Can you teach yourself new material by reading from a book or posting questions here on GMATClub?
-How much time do you have?
-What is your target score?
The problem with most prep classese is that they group high scorers with low scorers. Also, they plow through the material whether students are ready or not.
There are a couple of things I've stated over and over. One is that GMATPrep is golden. Another is that these gmat prep courses are NOT a magic wand. Just because you take a course does not mean your score will go up. The only guarantee from a prep course is that you'll be out about $1,000.
So ultimately it's up to you.