A recent article in the Personal Finance section of the Wall Street Journal talks about the math of clipping coupons. I think there are a couple of of important points here. One is that a knowledgeable (and aggressive) consumer can save a lot more money than is probably obvious to most people, because most people don't walk through the calculation of how much they're actually saving.
The second critical point is that savings are rarely considered from an after-tax perspective. Despite Ben Franklin's assertion, a penny saved in this case is really not the equivalent of a penny earned. Setting aside the peculiarities of our income tax code for the sake of simplicity, consumers in the 33% tax bracket must actually earn an additional 50 cents for every dollar they save. Saving $20 at the grocery store each week really allows you to forego an additonal $30 in earnings that would have been required to spend that $20. In other words, consumes tend to undervalue the impact of the amount they're saving.
Interestingly, more often than not my wife and I find that the coupons we're prepared to use at the grocery store do not save us enough to match the cost of the equivalent store-brand product, so we do not end up using the coupons. Nonetheless, the article raises valid arguments about how to go about determining if a "frugality initiative" makes sense for you.