Below is an article written by Fooducate- a very reliable source on nutrition. Here is the official link as well:
http://blog.fooducate.com/2011/12/05/whats-in-muscle-milk/

We’ve been asked by Fooducate community members about Muscle Milk, a supplement that is popular with body builders and athletes. Normally we don’t venture into the supplement realm, but recently we’ve been seeing Muscle Milk in supermarkets and in the hands of casual athletes, so we decided to take a look.

The idea behind muscle milk is to emulate human milk without using any milk in the product. It is a mix of proteins from milk, vegetable oils, flavorings, vitamins, and minerals.

If you are not a top athlete, there is nothing in muscle milk that your body can’t get from alternative sources.
If you are an athlete that needs a boost, consider the following:

– Muscle milk is a heavily processed product.
– It contains heavy metals in quantities that are borderline unsafe.
– It is artificially sweetened with the worst of the bunch – Acesulfame Potassium, a sweetener that has been linked to cancer. And Sucralose too.

If you take into account that most Americans get more than enough daily protein, there really is no reason to subject oneself to cool sounding products that could potentially do more harm than good.