Are work web blockers a waste of time?

Depending on which side of the business fence you sit, you could either argue that it does no harm to cut the workers a little internet slack, or that those internet slackers are costing your business money in terms of lost productivity and wasted bandwidth. But is there evidence to suggest expensively acquired web-filtering tools actually do any good, for either the business or the increasingly harassed employee? Could they even backfire on both?