While the levels for any individual book may not (and most likely will not) be accurate on at least one or more of the scales used, I have found them (all scales taken together over 150+ books read) to be meaningful to track progress in the case of my dd. I find that the grade level equivalent and the guided reading level are correlated with the book's length, the sentence length, and the number of non-repetitive words. The lexile measure is more closely correlated with number of difficult-to-decode words in a text.
DD is a late reader compared to children in this forum. So maybe the scales are only valid for average to slightly above average readers. My guess is that once dd gains reading stamina, she may be able to read much higher levels as her decoding is pretty good and her comprehension is way up there. Anyway, at that point, I may stop using the various reading scales as well. For now, I do find them very useful.
