hi
i think self studying is the way to go - IF you can accurately gauge where your starting point is. to do this, i would first take one of the GMAT preptests from mba.com, and record your score (also write down your section scaled scores). if you see that your 1st practice test score is reasonable close to your target (lets say you score a 600 on the practice test and are shooting for a 700), then you might benefit from studying by yourself. on the other hand, if your first test score is a 450 and you want a 700, then you might benefit more from a structured class than by studying by yourself.
however, there are always exceptions - ive seen people on these forums improve greatly just by studying on their own, even if their original starting point was low. it really depends on the individual. but if you do choose to do it on your own, you can always ask questions and read other people's comments on these forums, so you "technically" won't be alone