I like MM and have found it to give a good basis for my monkeys. That said, dd, my non-mathy child, took a bit of time going through it and had a bit of a hard time getting some of the concepts down. Manipulatives and games helped to get her through the tougher parts. Now that we're about 1/2 way through 2A, I glad we pushed through level 1 because now she understands the whys and hows.
Ds, on the other hand, has moved much more quickly through 1A and has just started 1B. However, I'm anticipating that by summer he'll be in 2A. He understands the basics much better and seems to acquire the skills more quickly than dd.