For my part, we're loving the "Challenging Word Problems" and "Intensive Practice" books instead of the regular workbooks. They're much more challenging and thought-provoking. They seem to me to go "deeper, not just faster" with even arithmetic.
I'd also recommend getting the homeschool teacher's book instead of the textbook if you're only going to get one or the other. I find the textbook to be pretty <meh>. Color pictures, but not much real teaching. The homeschool teacher's book, on the other hand, has given me some good ideas for introducing material and some uses of manipulatives to reinforce material that I wouldn't have come up with on my own.
Of course, I'm an English-type person. If you're very math-oriented, you might not really need the help like I do.
Enjoy!
