BMI can sometimes be useful when you're looking at populations. In terms of individual people, though, it's crap. I mean, sure, there are a good proportion of people who will be healthiest at around their "ideal" BMI. The rest of us, though, just aren't built that way. Hell, to be in the "healthy" BMI range, I'd have to be at under 5% body fat; and my body seems to be healthiest and happiest in the high-teen-to-mid-20s "love handles and a bit of padding" range. I have "obese" friends and relatives who are completely healthy in terms of blood pressure, cholesterol, etc; and I have skinny friends and relatives who have to be on BP and cholesterol meds.

I mean, sure, BMI can be described as reasonably accurate on average. But ignoring how individual people depart from the average, or insisting that they must be average to be healthy or happy, is just a steaming pile of ass-haberdashery.