We've been using Right Start Math since the K level and we are currently finishing up D. RS is fantastic in the way it introduces concepts through the use of manipulatives. It is rigorous, though, and my daughter hates the worksheets, but she loves the games. I find that I adapt here and there, and I often split the lessons in two. But by and large I find it easy to use, and my daughter's math skills--especially her mental calculation--are excellent and I credit much of that to the RS approach.