I heard such great things about this program, and my kids love reading.
We used this curriculum in second grade, and though there are some wonderful books, I quickly discovered that some of the books are inappropriate for the age they are used for.
One of the second grade books had nude photos. In doing research in other grades I have discovered it is a common problem with this curriculum. Our kids could both do the same core, but each core takes a long time, so if your kids are not close in age expect long school days as the teacher.
If you have enough time to properly research each book in every core you buy before teaching, then you might not have an issue with this curriculum, but we simply did not have that kind of time.
Some positives of this curriculum are that the kids did retain what they learned for the most part, and there are some great books in the program. I like some of the books, but not the program as a whole.