Whenever I have a particularly good (or bad) experience with a product, I let the manufacturer know. If I have had a problem, someone else has probably also had the problem and didn't take the time to let them know--so my comment bears a lot of weight. When I really find a product that I like, then I let the manufacturer know that I want this product to stay around and that I would really appreciate any coupons they could pass along--so I can buy more!
There may be a small price of increased "junk mail" and "spam," but I find sorting through a few extra envelopes worth the numerous free products and $x/x coupons that I have received because of these comments.
Today I used a $1/1 any Farmland product Q on a $2.99/lb package of Bacon--I received an entire page of $1/1 and $.55/1 Q's--that is a dollar that I would have spent had I not let Farmland know how much (and how frequently) we enjoy their foods.
$40 WalMart GC AutoVantage