I used it when my oldest was in 6th grade. I liked the premise of it - teach the prepositions to get them out of the way so the student could find the other parts of speech easier. However, after spending the entire year with it and doing well on the tests, my son didn’t actually learn much of anything. He actually asked me after that year what a noun was! 
I still like the preposition first method, but I don’t like the way EG teaches prepositions, which is pure memorization. Many prepositions can be used as other parts of speech as well, so you can’t just memorize; you have to know how the word is functioning in the sentence. So I’ve become a non-fan after that.
