I took four AP tests, European and US history, Government, and Economics. I did miserable on the Gov and Econ ones, but I knew it pretty early on in the year, as the teacher was terrible and I had zero interest in economics. I passed Euro and US History, and it transferred into 12 units at my university, which was a huge help when graduating. It basically got me out of college a semester early.
Those two AP classes that I actually passed are what prepared me for college though, the writing skills alone made most college level essays a breeze.