I'm not talking about boycotts in general, like say the Montgomery Bus Boycott, because we know those can work. I'm talking about "Company that sells product made me mad" boycotts.
Whether it's past ones like the infamous Call of Duty: Modern Warfare 2/Left 4 Dead 2 boycotts, or more recently like the brewing chud Pride Month boycotts (And if we're being brutally honest here on the flipside: The all-but-in-name boycott of that Harry Potter game a month or 2 ago), all of them seem to universally fail spectacularly and not impede their targets in any way.
Work, as in changing the way the company operates, probably very rarely if ever. However, there is another way of looking at this: action enforces identity. Boycotting seems very good for this purpose because it takes almost no effort from either the boycotter or the organization or movement calling for one. Yet the boycotters end up feeling like they did something for the movement, which makes them identify with it more.