"....blogs give employees and customers a dedicated forum in which to vent their concerns and anger - an open invitation for bad PR. Furthermore, blogs can undermine the carefully controlled release of information from a company. A careless entry from an employee can be particularly dangerous for a publicly listed company intent on keeping market-sensitive information out of the public domain".Examples in article include: Supermarket-sweep-up - presents a bigger picture of what Tescos are doing, and their effects on our society. And, 173 Drury Lane - a critique of Sainsbury's supermarket. Also see Tescopoly - every little hurts, SchNEWS - a direct action protest page.
Having said that read (but not with reference to above) - When fake blogs attack - which considers the immediacy of a first-hand accounts.