I've used Right Start Levels A and B. Now we're doing that in addition to Math Mammoth--as DS1 was very eager to learn multiplication. Right Start Level A was perfect for us--especially as DS1 did not like writing when he was in Pre-K/Kindy. It is very hands on--with very little written work. When he hit 7, he seemed to blossom into loving writing--and actually wanted more pencil/paper work, which is why we added Math Mammoth.
DS2, is 5, and is loving Right Start A as well.
I've looked into SIngapore as a possible choice for DS1 for next year, but I think at this point (well, at least for the summer), we're going to continue with Math Mammoth as he seems to really "get" it.
I definitely agree with those who say that you might love a curriculum, but your kids might not.
So, if you can, try and find some sample pages and play around with your kids. One thing I loved about Math Mammoth is that she gives you 300 different sample pages so you can definitely get a good idea about it. Also, it was super cheap... and you buy the topics you want. RightStart is very expensive IMHO, but my kids have loved it--and they've really seemed to get the concepts. The variety of manipulatives also was a big plus for us. My kids still want to "play" math. The math games too are great.