I have used MUS extensively with my students for whom I do their complete math program (meaning they have little to no time in the regular ed. math class). I love MUS ~ I find it effective, straightforward, and easy to teach. However, it does NOT match the 6th grade state standards in CT. MUS is arranged topically - each book covers one school year (approximately 30 lessons, each designed to take 5-6 days) and is set up so that students cover all aspects of that topic during that time. The first book, Alpha, covers all addition and subtraction facts as well as other basic first grade skills such as shapes, place value through hundreds, some time, etc. The second book covers additons and subtraction with regrouping as well as other skills. The third book, Gamma, covers all multiplication topics, from basic facts through multiplying 4 digits x 4 digits. As they move through the facts other skills are introduced as appropriate - with x2 converting from quarts to pints is covered; with x4 counting by quarters is covered and converting from gallons to quarts. Word problems are included throughout.
There are placement tests available on the web site; however, I typically start my students in Gamma if they don't know their facts. Sometimes I can do Gamma and the next book, Delta (division) concurrently; sometimes I've covered Gamma and Epsilon (fractions) together. It takes some work and some modifying from their basic expectations, but it is doable. The hard part is that the typical 6th grade curriculum covers in one year/one book with typical publishers like Houghton-Mifflin or Scott-Foresman, what MUS covers in 3-5 books.
You can print out the scope and sequence from each book on their web site; I suggest you do that and compare them to your state standards to see if it will work for a complete program or more as a supplement (which it's not designed for and is hard to do, imo.)
I've used MUS extensively for 5 years or so now, and I have quite a few materials developed to supplement it, primarily extra word problems. Feel free to e-mail me with more questions.
Jenn