I have used the Horizons Math program for grades kindergarten, second, and third grades and think it is one of the best and most challenging homeschooling math programs available.
The colorful and fun workbooks introduce new concepts, but keep a constant review of lessons learned.
I would not suggest it for struggling students, but it is great if you want to give your kids a challenge.
My kids never get bored and are always being challenged beyond what I have seen in other mathematics programs.
We will order it again every year through 6th grade. 🙂