Back in 97 I received college credit for any exam with a score of 3 or higher. I would check with the colleges you are thinking of applying to (junior) or the college(s) you've been accepted to or will attend (senior) to see what kind of credit they will give you for a given score on a given test.
Personally, it was worth it to me. I was able to AP out of my humanities and social sciences requirement in engineering. I had to take one english class in college, and the rest of my courses were math/science/engineering. A 5 on the AP US History test got me 6 hours of history credit... US history before the civil war (3) and after (3). A 4 on the government test got me out of 3 hours of poly-sci (us gov). A 4 on the english lit exam got me out of my basic honors english course (3 hours).
So, I got out of 12 hours, which is almost a semesters worth of classes. 12 hours would have cost me around $1300 at the time. It was definitely worth it for me.
R