We have used the curriculum for 7 years now and I am in the process of changing to Teaching Textbooks. I don't want to be seen as trouble maker but, Math-u-see has it good points but many bad ones also. My children have grown very tired of the repetitive nature of the work and found the concentration on math principles to be difficult.
There is not variety like other programs with the math principles. My children have taken a long time to a complete books because of the nature of the program. This in itself frustrates them as they feel they are getting no where quickly.
I think if you were to use it with other curricula, it is great as you learn solid math concepts. I have used the keys to fractions decimals and soon to use percents along with it to get my children up to scratch with where they should be to enter into teaching textbooks.
Before changing to Math-u-see I very much compel you to think lengthy about the decision as using other programs hand in hand with can become quite expensive but the up side is you will have a complete math system teaching topics in depth. I for one am not sticking it out any longer as my children are tiring greatly of math.