I am going to guess that the Mathnasium experience varies greatly from location to location. We tried the one closest to us requesting "enrichment," and did not think the learning justified the high cost. They seemed to be simply assigning grade-level worksheets and not allowing our DC to move quickly though the material. DC enjoyed it, but DC is social (and liked the lollipops they provided)! I don't think it HURT DC, but I can't really say it helped much, either.
That's really interesting.
Around here, they're very specific about their service being focused on what the child needs. They told me that some of their kids do their homework at the Mathnasium, while others follow Mathnasium materials, and still others do what Mom or Dad sends with the kid (in our case, DS does Singapore Math there).
DrummerLiz, my advice would be to quiz the whoever runs your local Mathnasium about their local policies.