My daughters love them. I consider them to be fine for free time and in moderation. Mostly, I just see to it that the girls are reading other things first and then if they have time left over they read the MT books.
I just think it is like eating crackers - they are tasty, but not very nutritious :shrug3. As long as they are getting a good diet, though, a few crackers are probably fine. If I saw, though, that real nutrient-dense foods were being replaced by crackers, then it would be time to reevaluate.