DS6 liked it a lot when we first started homeschooling, when he was picking up basic concepts. We started with 1a, the Kindergarten would be too basic for kids with any sort of number sense (we use it for DS4 and it's perfect to introduce numbers), the one thing I really love about the series is the way they relate the numbers. They really stress to the kids that a number is the sum of its parts (i.e. 5= 2+3, 4+1, 0+5) and pretty much all of 1a is devoted to teaching that concept. Our problem with it is that while DS isn't self-taught in math, he can project concepts out pretty far once he's shown and even just into 2a we were finding that he had already figured out the concepts and was frustrated at having to practice bits and pieces at a time. If you do go with Singapore, I would suggest using Intensive Practice and Challenging Word Problems instead of the regular workbooks.
There's another series that we tried in the beginning- Miquon Math. It was very interesting, it's based on research done in a experimental classroom in the 70's and definitely has a "off the beaten path" feel to it. If you have a homeschool store near you, it might be worth checking out. The workbooks are pretty inexpensive, like Singapore, but the teacher's manual is a must (some of the exercises are completely mystifying without the explanation) and it's a bit pricey (~$20), though it's just one book used for the whole series.
