I have been using AoPS for almost two years now, and it has taught me so much more and in depth math than school ever could. Every formula is proved and the classes are very student interactive. It teaches you to problem solve and think critically.
My only two warnings would be that:
A) It's extremely hard, and no piece of cake. The site details that a child who gets straight A's in school shouldn't be sad if they get a C in an AoPS class.
B)It does not necessarily align with school standard, so be warned.
Overall amazing website!