My daughter's school uses i-Ready. It does give differential lessons to each student based on how they've done on the tests. It even occasionally circles back in areas you were maybe just "ok" in. The problem I've seen (based only on her reports so not even remotely scientific) is that if you fail a section, it gives you the exact.same.lesson again. It doesn't try to show you the same material in a different way. It just cycles you back through the same exact thing. Additionally, she reports that "all" of her fellow classmates hate the time they spend doing i-ready. I suspect it's not very engaging, there's no group/social interaction, etc.