You are sitting down with family and friends in a supposedly smart restaurant - and the food is disgusting. What do you do when the waiter asks if everything is ok? Like the majority of the population, you probably half-smile and mumble something acquiescent. Wrong! You should always complain and always explain! A complaint without explanation is worthless. Ranting and raving is bad; controlled complaining is good. Don't just walk away, let the people who caused that feeling know why. In all cases you stand to gain a lot more than ranting, or not saying anything at all.
Used availability for Mel Stein's How to Complain