Actually, I went from regular classes straight to AP; I was recomended for AP right after my freshman year. Difficulty and structure of classes will differ between schools as well as individual teachers, but the main difference I observed, and enjoyed, was that there was greater emphasis on problem solving methodologies in AP instead of simple memorization, which is a characteristic of many regular courses. I highly recommend AP U.S. History, it can be tough as nails but it is fun and will give you some significant analytical skills.