From that link:
1. Body Mass Index (BMI)
BMI is a reliable indicator of total body fat, which is related to the risk of disease and death. The score is valid for both men and women but it does have some limits. The limits are:

It may overestimate body fat in athletes and others who have a muscular build.

It may underestimate body fat in older persons and others who have lost muscle mass.
BMI is a joke unless you meet the definition of average IMO, which is probably very few people. Granted, we're (Americans) a fat and lazy lot, but still, very few meet the definition of average. Which is what the BMI is designed for. I'm sure it'd be difficult to come up with a scale that accurately takes into account bone and muscle mass that one could do themselves without having a professional take the measurements. Either way I think too much stock is put into the BMI and no one EVER recognizes the two above statements about estimates.

Mine is 31.9 and I've never met an obese person who could run for 80 minutes with bursts to full exersion thrown in every few minutes in a rugby match... Go figure.