Well successful is something that results in change that was desired by the boycotters. So for example if you objected to the Tesco that was built in Gerrard's Cross (one of the poshest towns in the country, no shops open on Sunday), not going there after it opened, and having close down through lack of custom.
I'm not sure if the Nestlé boycott has been successful in the sense that they have changed at all.
But things like Gap can work as they sell to a middle class customer base - though I wonder if their changes were a response to an actual boycott (fall in sales) or more as a damage limitation exercise to stop the bad publicity.
Consumer behaviour has brought about change in respect of things like battery eggs, and there's definitely far more 'freedom food', 'free range', etc. meat on the shelves.
Food marketing worked for decades but I think these days people now realise that what Bernard Matthews sells is shite, though I'm not sure that Danish bacon has been seen through yet.
I think The Sun's sales are still suffering in Liverpool.