We started my 10 year old in Gamma this year, and I am so glad - he was so close to being Delta, but not quite, but we decided to back track anyway just to be sure. I really want him to "get" what he is working on. It is easy for him right now, but I am sure we will have to try harder once we get into the book (multi digit mult instead of just learning the tables).
I am so happy with MUS, I'm going to use it w my DD too, just because. :)