All it is, is weight to height ratio, so if you have a lot of muscle or whatever it's not a great indicator. Body fat percentage is better, a lot better. B.M.I. is something that is used often as a general indicator of being over or under weight, but it does not take anything else into account. Someone could have a fine B.M.I. but their bodyfat percentage could be too high, and vice versa. So, it is and is not B.S. at the same time, haha. Hope that helps a little. You can buy a body fat eletro thingy at walgreens and that would help you more (it's more acurate than a caliper/pincher). Just make sure that you are hydrated when you measure it.
I took a nutrition class in college and even my professor, who was a licensed practicing nutritionist said that the BMI system was not very accurate, especially for athletes or people with more muscle mass.
I think BMI is a dreadful tool for an individual but has some merit as a very general demographics tool for things like comparing obesity rates between countries and the like.
I also have no use for the gimmicky fat-measuring devices using infa-red, electrical resistance etc. Onn the same machine you can get readings that are all over the park from day to day...sometimes depending more on the humidity in the room than on your bodyfat.
An EXPERT with calipers (no, not your gym trainer) or an underwater measurement are the best ways to go. Don't try calipers yourself, it;s too frustrating.
Some of the formulaic methods that include frame size considerations like wrist diameter, shoulder width, pelvis width, ankle bone width are really pretty good. Look around the web for them.