This has been my experience with the classes I validated or for which I received advanced placement.
Chem - You will take a chem test during the first week whether you want to or not. I did not take AP Chem in high school and sufficiently bombed this test. However, if the rest of your grades are high, you will more than likely receive advanced chemistry as a class. This class is for those that either scored higher on the placement exam, but not enough to validate, or those like me that never took the necessary courses in high school but were deemed smart enough to be able to handle the course rigor. That being said, I highly recommend not validating this course. The work is significantly harder for very minimal gain. The general course is much more structured and graded assignments are easier. You still take the same quizzes, WPR's and TEE's, but I felt much more prepared for them in gen chem than advanced chem. (To explain, you must receive an A- or higher at the end of your 6 week grades in order to stay in advanced chemistry. I had a B+ and was subsequently resectioned to gen chem).
Math - Like math, you will take this exam whether you want to or not. I took AP Calc in high school and received a 5 on the BC test, so I felt the exam was sufficiently easy. I highly recommend validating this course, as you will only be required to take 3 semesters of math (multi-variable calc, diff eq, and stats) as opposed to the 4 that those in basic math take. Additionally, you can validate statistics, but you are required to take an exam to validate around spring break of your plebe year. Although I received a 5 on that exam in high school, I didn't remember anything from the course and decided the better course of action would be to retake the course rather than limp through another test. If you were one of those geniuses in high school that was somehow able to take multi-variable in high school (I know a girl who did it) you will take diff eq in the fall. I don't know what you'd take in the spring in that case, but at this point I feel that I'm addressing such a slim minority of readers that they can figure it out for themselves.
Psychology - YOU CANNOT VALIDATE PSYCHOLOGY. I know, it sucks. I received a 5 on the AP course in high school, but West Point feels that it is a necessary course for young leaders, even if this is your second year in a row taking the course. I was placed in PL150, advanced general psychology and successfully avoided studying for an A in the course. There you go.
History - History's a little weird. First off, there is no validation exam. Based on AP scores alone, you can validate plebe history, they just take you aside and have you sign a piece of paper saying you want to validate. I took AP US History and AP European History in high school and received a 4 and a 5 respectively. This allowed me to validate all of plebe history and take History of the Military Art, the core history course for Firsties. This is one of the first years they've done this, and it was due primarily to the large class size of 2014. Most people that validate plebe history will take 2 of the Yuk social sciences courses (either Dirt, American Politics, or Economics). If you took one of the two of these AP tests, you will be placed in the opposite Plebe history course, advanced placement depending on your score (if you took APUSH, you will take Western Civ, if you took AP Euro, you will take US History). The only way to completely validate plebe history is to have taken both of these AP Courses (to my knowledge anyway). Definitely validate this if possible.
English - English is pretty much a volunteer validation. You must take an additional test during Beast, which features you writing about a poem you don't understand for a few hours. I took AP English in high school and got a 3 on the exam and was asked to take the test. My friend didn't take AP English in high school and volunteered to take it and validated it with me. If you validate, you will take EN102, Literature, the fall of your plebe year and PY201, Philosophy, during your spring semester. Those that don't validate will put these courses off one semester and are forced to take a tedious composition course. I highly recommend trying to validate this, whatever your English background might be. This is a huge time-saver.
Physics - This is a Yuk course, but I'll talk about it now. You can validate this course either by AP Scores or taking validation exams. You must have received either a 4 or a 5 on AP Physics C Mechanics to validate PH201 and a 4 or a 5 on AP Physics C E&M to validate PH202. AP Physics B scores don't count. I received 4's on each of these so I validate the course completely. I highly recommend validating this course, as it is a renowned GPA killer for yuks. That being said, if you plan on majoring in some field that requires a working knowledge of physics, take this course over again. A solid foundation can't hurt.
This covered my experience. Let me know if I need to elaborate anything.