Check out this thread from last summer.
http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/195981/1.html

Personally I don't like the books at all. My son would hate them. Why make something so simple as math complicated with so many words. The books are VERY Christian IMO and defiantly have an Christian agenda. And some of the math is of questionable mathematical accuracy. I've lost the link but you can read several of the books online to get a taste.

But many people seem to love the books including some on this board.