We are using Singapore, but my DD is only in Earlybird 2. She really likes it -- it is bright, fun, and no busy work.
I feel very comfortable with Singapore. I did quite a bit of research, and Miquon is the only other program I would consider. They are both very solid and provide an excellant math education. (We may end up using both if either of the kids need more reinforcement)
Saxon would drive me bonkers. I don't think that the k-3 Saxon program is very good because it has too much drill and no thinking, so kids get the idea that they don't think in math. Singapore does things in such a way that the kids have to use their brains and think, not just spit back answers. Have you been to the WEB site? There is a nice message board there, and other refuges from Saxon.
Here is an interesting WEB site about math education -- not much about Saxon or Singapore, just lots of info about how bad math education has gotten in the US.http://www.mathematicallycorrect.com/