We did part of OM 1st grade, and while there were some things I loved about it, it ultimately wasn't a good fit for us. I tried to do it "open and go" and I was caught unprepared a few times as far as having certain supplies on hand, etc. So if I were to do it again, I would definitely try to read ahead for the week to get a feel for what is coming up. But that was mostly laziness on my part.
I think it is actually fairly straightforward AFA prep work goes.
I wish I could afford to buy each grade's syllabus just for the science. I absolutely loved the science in 1st.