Well, we tried MobyMax with a free account and it definitely should NOT replace curriculum. I tried it with both DD (3rd grade, 5th-grade-ish math) and DS (5) so we have been doing it at two levels. At neither level did it do a good job teaching concepts. It "introduces" new things with a very brief and sometimes simplistic explanation that didn't really impart understanding of the concept--just the procedure to solve the problem.
And with my younger child, it was easy for him to stumble accidentally into the right answer just by try-and-fail methodology because as soon as you happen upon the right answer, it moves on, instead of waiting for the child to tell you he's done working.
I'd say it's equivalent to IXL--ok for practicing procedure, some fun features, but it's not going to teach the concepts kids really need to know.
AoPS doesn't offer a curriculum between grade 4 (Beast Academy books up to 4C, with 4D coming "later this year") and Pre-Algebra.
