We finished the Singapore series through 6B this year and I can't say enough about them. Many pre-algebra concepts are covered through 6B. We only had/have a little hole filling to be really algebra ready. They've been a great and challenging fit for my math gifted 3rd grader. He just tested at the end of 8th grade in achievement testing (in an open ended test - not a 3rd grade specific test)
The placement tests can give you some ideas about what it looks like. Very few curriculum work well for us, but this has been great. I actually feel it slowed us down through elementary math because of the challenge level. And they are cheap enough that if you order just a text and a workbook and it isn't a fit, it isn't the end of the world. Unlike many other math texts! (I currently have $$$ in math curriculum for next year laying around the house I'm not sure is going to be a fit. I'm just at a loss on what to do next)
