I have purchased and used Math Mammoth for my 4th to high school tutoring business. I use the summer workbooks year round supplementing students who may need more time in a concept or that wants to move ahead.
I love the depth of the curriculum knowing all material for a grade is covered in them thoroughly. I purchased the PDF version and simply send them to a local printer to get the books I need. Math Mammoth is very helpful if any bugs are found (which I have not found any) or updates to packages.
A great home school or tutoring program for me especially for Summer Brain Drain....it really keeps students at grade level if the student diligently works the grade finished in the spring all summer.