If bmi says you're obese, you very likely (~95%) are. The problem is that bmi only catches about half of people who are obese.
How about other categories, in either direction? I seem to remember it to be a lot less clean cut at the normal/overweight boundary.
I haven't been down in the normal BMI range since 8th grade. And I was measured at 6% body fat my senior year in high school! BMI has always run several points high for me. For that reason, I've always liked waist to height ratio better.