I'll make a partial exception for calculus. Because of the extensive prerequisites in majors such as engineering, having cal I or, better yet, cal II out of the way can be a tremendous advantage to an incoming freshman. Add to that the fact that the nature of the subject makes teaching to the test much less of a concern.
With that exception noted, I never saw that strong a case for AP. On the whole, I suspect that college level material is better taught by college faculty, particularly given the test-prep approach of many of the AP classes. If anything, I'd like to see more anti-AP programs. Instead of giving college credit for high school courses, give high school students more opportunities to take college courses (either on site or through distance learning or some kind of independent study). There are certainly precedents: even back in the dark ages, I was in a program where as a high school senior I could attend the local college half-time. With the advent of distance learning, email and digital media, the argument for AP has only gotten weaker.
At this point, I should segue gracefully into a discussion of this paper (via TNR) on the impact of AP courses but to be perfectly honest, I'm in a hurry so I'm just going to give you the abstract and let you all talk it over amongst yourselves:
The Advanced Placement (AP) Program was originally designed to provide students a means to earn college credit and/or advanced placement for learning college-level material in high school. Today the program serves an equally important role as a signal in college admissions. This paper examines the extent to which AP course-taking predicts early college grades and retention. Controlling for a broad range of student, school, and curricular characteristics, we find that AP experience does not reliably predict first semester college grades or retention to the second year. We show that failing to control for the student’s non-AP curricular experience, particularly in math and science, leads to positively biased AP coefficients. Our findings raise questions about recent state policies mandating AP inclusion in all school districts or high schools and the practice of giving preference to students with AP course experience in the university admissions process.