We made the switch last year from Everyday Mathematics and Horizons curriculum over to Singapore Primary Mathematics, US Edition. I did this after some extensive research and lurking on a lot of math-y people's blogs. Singapore Maths is the sort of math that good public school teachers wish they could teach, but can't. *So* far, I have not heard anything negative about it except for the fact that it is difficult to "differentiate instruction" for this curriculum.
The advantage of this curriculum is that... well... I'm learning something with the children. Perhaps I've just missed a LOT of math because I moved often, but now the subject is presented clearly and coherently. I can even do some of the problems in my head. And why? Because this program trains you to do it that way.
Disadvantage? Tricky wording may make answering the questions difficult. Sometimes there are two or three steps to a problem, and children stop part of the way through thinking they've answered the question. It makes for a bad test result, but good training to *read* the question and answer the question, not what you *think* the question says.
Another oddity is that it is definitely written by a non-native speaker of English. How do I know this? I'm not quite sure. But do you say things like, "Mrs. Bates bought 5 pots of plant," in general conversation? I would say, "Mrs. Bates bought 5 plants," or "Mrs. Bates bought 5 potted plants," or "Mrs. Bates bought 5 pots with plants in them." But not five "pots of plant."
The curriculum comes in an A and B set for each grade. We have completed 4A and are going to do the lesson pictured in the textbook on Monday. There is a textbook and workbook. We do the textbook problems on a dry erase board and write in the workbook just on the off-chance the state wants to look at my records.
It's a secular curriculum, so if you're looking to learn multiplication with Jesus and the fishes, this isn't for you. I am a reasonably moderate sort of guy and have found nothing objectionable to date. If it helps, all the children pictured in the books are well-groomed and dressed in two-decades-ago appropriate clothing.
I bought my stuff from Sonlight, but it's also available from Rainbow Resource and several other homeschool suppliers. Make sure you get a teacher's edition, even if it is technically not written by the Singapore Math people. NOTE: every now and then, there are glaring errors in these teachers' editions. Mine are marked up with wrong answers. A good rule of thumb is that if Emperor consistently gets the same answer, and it doesn't match the book, it's time to say hello to Mr. Calculator and scrap paper and see who's right.
Usually it's Emperor.