I've used MMM for four years now. Next year I will have four students using K thru Level 3. A close homeschooling dad friend (who taught 7th grade public school math) chose it for his children after attending our local book fair and talking to ALL the math vendors. Back then when I was just starting out and had no idea what type of math to go with, his opinion was good as gold.
MMM is very different in it's approach to teaching math. Right away he has students thinking algebraically. I'm no expert, but it seems this will really benefit them later on down the line. There isn't a big emphasis on memorization/drills at first. I encourage you to call the author. He is willing to talk with you, can help you understand the benefits of teaching math this way, and share tips on teaching. His own children were taught this way and were/are successful students.
I truly believe it is a good method. It is a bit teacher-intensive, but you learn how to manage it. I've noticed my DS7 especially enjoys it. I hope this is helpful. You can PM me if you'd like.