Quality matters. ON is mostly whey concentrate -- the cheapest form. (You can get 10 lbs of whey concentrate at the same price at Protein Factory). Muscle Milk is a superior blend, plus it has a great flavor and mouth texture.
I use PF to make my own chocolate (Nestles Quik) and Strawberry. I get the BIG BLAST flavored (Tropical orange mixed with Sunny Delite is evil) and use Muscle Milk banana and vanilla caramel. That way, I don't get sick of the same thing.