I was having trouble with my daughter doing MUS so we started doing MEP. I also have her practice math facts on xtramath.org once a day. She is slowly improving, but it is pretty slow going. I just don't think she is number minded. My 10 year old does fine as long as we review or supplement with MUS once in a while, but she just wasn't getting it.
My daughter is only 8 so we started her in Year 1. It has helped her to have different ways of looking at things and using lots of different types of manipulatives. She wants to try teaching textbooks next year because she really enjoys doing things on the computer, but if she has trouble at all I will just jump back in with MEP.
I have looked at Math Mammoth a bit as well and we even used it once in a while for fun. If you are looking for a less teacher intensive program, it would probably be a great option. MEP requires you to do most of it with them.
Life of Fred is also a fun option that we supplement with, but I don't use it as a stand alone curriculum.