Really really useful.
I load mine up with whey, bananas, natty PB, yoghurt, honey or whatever else I fancy and get a smooth yummy weight gainer out of the spout.
I think mine is actually a smoothie maker but the same principle applies.
Good for making fruity smoothies, protein smoothies, soups.
Turns out is not so great for making butter icing (my housemate - not me) There is no room in my diet for butter icing. If I need a vice it will be Jack Daniels!
But I digress - food processor = good plan!