I've seen kids later have issues after having done SM. It seems that they just gave the curriculum to the kids and the kids did it w/out really getting the SM way of thinking. So at the higher levels, they hadn't developed the mental math abilities etc to manipulate numbers. Someone had written a great post at WTM curric board about SM saying that if you're not doing it as in the thought bubbles, you're not doing it right. Plus, as you mentioned, Primary Math doesn't have the drill in it. In Singapore, teachers handle the drill and the parents as well. But people get SM b/c it's cheap and don't buy the HIGs which I think really help in learning the SM way of thinking.