I didn't do well on my AP US History Exam when I took it, not because I didn't know the material obviously, but I didn't finish the essay parts of it. I started getting cramps in my left hand, sad case of irony when you're getting hand cramps from writing so much and your a writer as a hobby.
Also, I agree, I only took one other AP exam and that was for English, the US History was a heck of a lot easier than the English exam. It probably helped that I liked learning US History which is odd because I always hated anything related to Social Studies.