BMI is a pretty decent measure of fat percentage for the vast majority of people, it really only fails for people who have far more muscle mass than most people (body builders). It is highly correlated with health outcomes - people with a BMI over 30 (obese) have significantly higher risk of death from heart disease, stroke, diabetes, and cancer - this is true at a BMI of 30 and it only gets worse from there. 40% of americans are obese. It's not overblown - if anything, I think most people don't understand just how big of a health crisis it is.