I have used Math Mammoth at various times with all three of my children. Each one learns differently from the next (ADHD, dyslexia, math hater) but all three found Math Mammoth easy to use, not too much in the way of problem solving, and very clear on the explanations.
I like it because, besides the reasons given above, it fits our small budget beautifully and I can purchase and download only what I need - instantly. We have used the Light Blue series the most, but when a child needed extra help with a topic it was so simple to buy just the topic they were stuck on and give them that extra help!
Maria's newsletter is always interesting and helpful, too. I would suggest signing up even if you're finishing out your year with a different curriculum so you can see just how talented this lady is at making math understandable.