Ironically, it is because I was never good with math, that I have tried my darnedest to find a program that would help my son to not only learn the subject, but to LOVE it. If you aren't sure, try using Singapore to supplement first and see how your children like it. It isn't expensive to give it a try. That's what we did before we switched over, tried it during the summer, then went with it full on just this year. From what I can see, these workbooks, which are meant for review and supplementing their coursebook, are outstanding.
All answers are in the back of the book.
The enrichment books are consumable. As for the measurement issue. To me, that is not a big deal, my son has been learning both systems of measurement from the very beginning anyway. As I see it, it is more important for my son to have a solid grasp of the metric system, since that is the system used in the fields of science and technology today. It's important to know both of course, but metric is critical for science.