AP classes are quite helpful for a number of reasons, in my opinion. Like thechibi said above, it's mostly about handling workload. It helps you get a handle on what will be expected of you in college.
They're a bit more intense, but it isn't like you're just handed a textbook and told to be ready for the AP test in 6 months! You may have to put forth more of an effort than a basic-level course, but the class is still designed for you learn and be taught - you're not just expected to figure it out all on your own.
Additionally, being able to "place out" of freshman level courses is generally a positive thing. For instance, taking Calculus I in a high school classroom of up to 30 students is tremendously better than taking Calc I in a lecture hall with 300!