BMI is a decent estimation device, but can by no means considered accurate. You can tell just from the equation that there are numerous flaws. The biggest one IMO is the failure to differentiate between muscle and fat mass. Muscle weighs more than fat space-wise. Therefore, a lean, muscular individual might have a fairly similar BMI to an obese person. For example, I am 5'10 and weigh 155 with an 8 inch drop. I am in the middle of the BMI range for my age/weight group. My friend is the same height, yet weighs around 180, all muscle with a 10 inch drop. According to BMI, he should be far less healthy, however, that is far from the truth. In general, BMI can be useful, and its great for people whose body compositions fall in the mean, however, it can be wildly inaccurate.