BMI is a good basic proxy measurement for getting a rough idea if you are too fat or whether the whole population is getting too fat.
If you can be more specific like body fat %, well then use that. True body fat % is difficult to measure as some of the posters have pointed out.
The big objection to BMI that people always bring up, is that if you put on muscle, your BMI will go up. Sure thats true, but most people don't put on a lot of muscle. It depends on your genetics. I know I don't.
When we hear that the general population has an increasing BMI, does that mean that there are more professional atheletes and body builders in the population or that people are getting fatter? Which do you think?
I look at my weight, how much flab is around my middle and how easy it is for me to run up hills, to gauge the effectiveness of my fitness program. When I am doing strength training, I lose more fat than I gain in muscle so my weight and BMI go down.
Most people that tell you that they are putting on muscle and their BMI and weight are going up are full of BS. They are probably just getting fatter.