Bought this a few months ago and didn't get to frequently using it up until recently due to the fact that I was polishing off another supplement... but man so far I am very impressed with what I am seeing.
Before anything, let me say that I do little if any excercise due to my schedule. I'm a thin dude that had some muscle mass but nothing really noticeable. I drink two servings per day, once in the morning and once just before going to sleep. After about a week I (and my gf) have noticed a definite change in muscle definition and even more reduced soreness at the end of a work day (can be physically strenuous). And on top of those benefits, the chocolate flavor also tastes great and I've seen people say that the banana and strawberry flavors are even better.
As far as drawbacks, there's the period of adjustment like all supplements in which a person would typically develop above normal levels of gas and there is a weird amount of fat in it, higher than any supplement I've seen.
If you're looking for a protein supplement, I'd like to recommend muscle milk!!