We've been using the Horizons curriculum for third grade for about a month. The children enjoy having their own workbooks with the nice glossy covers. I enjoy having a teachers' manual with lesson ideas clearly numbered and coordinated with the workbook pages. I've taught some of the public school curriculum (Everyday Math), and I have to tell you that I have no idea how the teachers in public school don't go on a screaming, crazy rant trying to figure which worksheet goes with which lesson and where all 30 of her kids' notebooks are. It takes a lot of planning and digging through manuals to teach. And that's without doing the games and other extras they recommend!
I suppose what I *could* do is to figure out my own scope and sequence, and teach that from materials I glean from the internet. I could figure that the boys ought to learn multiplication this year, and print a zillion worksheets and tests for them to do that. I am just not that organized, and I like having someone else package the whole thing together for me. And while I'm totally responsible for my childrens' education, I like that someone else with experience knowing what a third grader needs to know in math has been working on the books I'm using.