Lexile is only one measurement of reading ability. It's is based mostly on the difficulty of the vocabulary in the book. It's not based on the length of the book nor if it's interesting or age appropriate. Two books can be of the same Lexile level but be either entirely appropriate or not for your child. It can be helpful in finding more challenging material for your child, but I don't think it ever should be the only criteria for choosing a book. I can find you picture books that have the same Lexile level as 300 page novels.
Lexile is more useful in my option for early reader books. Books for average kids K-3rd grade. Once you pass a certain reading level I don't believe it's a very helpful criteria for choosing a book or setting up reading groups. My kids school did this years ago and quickly found it was untenable above 3rd grade. (We had many high readers and although it was a K-8 we didn't a have books in the school library appropriate for 4th graders that were at their lexile level.)
That said.. the question was "Does anyone in the group have a perspective on the accuracy/usefulness of Lexile measurements for these groups of kids?"
It can be useful to group kids in reading groups and I believe Lexile can be one of the criteria in early grades but it shouldn't be the only one. Relying too much on Lexile for choosing reading groups can miss important details. Lexile works fairly well for grouping emerging readers. But doesn't do a good job for those who pass a certain threshold.
Keep in mind that there is nothing wrong with kids reading books above or below their Lexile level. In reading instruction, teachers try to find that level that is "just right" to push kids a bit but not overwhelm them. But for "fun" reading I don't believe kids should be restricted in this manner.
