I read a lot (!) of reviews before going with AoPS, and the general consensus seemed to be that the kids that really liked it tended to be the highly visual-spatial, "start with the big picture and then derive the details" kinds of learners. It may not work so well for those that prefer to build up the details piece by piece to make a larger concept. One Mom perfectly captured the spirit of the comments I kept seeing, noting she tried AoPS with he twin boys, and her "whole to parts" learner loved it, while her "parts to whole" learner found it frustrating and unhelpful.
