For me, they work when I don't have time to cook/get a real meal. But one, two max per day should be the limit. As said before, it's just a supplement, and doesn't have enough nutrients on its own. Combine it with something.
Example: Brad Pitt was getting a protein shake and a banana as a post-workout meal when preparing for Fight club. But he was just trying not to loose muscle and look lean, not gain muscle.
Depending on your goals, it's more important to customize a diet you can follow, and use suppliments just when necessary. Good luck with whatever you want to achieve!