Some fiction authors have been ferocious copyright defenders because they fear that by letting even well-meaning fans make derivative "fan fiction" works, they jeopardize their ability to own their characters later.
It's not a simple decision: every time you consider legal action, there are both predictable and unpredictable ramifications. Perhaps "making an example" out of an infringer will establish your presence as a brand that must be respected and taken seriously... Or perhaps your clientele will be turned off by your humorless, bottom-line-centered approach.
The most important thing is to understand the things that make your product valuable to your customers. If you know why customers are attracted to you (superior quality, distinctive flavor or styling, or convinience, for example) you can estimate what parts of your brand are vulnerable to competitors. Those are the places you need to redouble your defensive efforts. As outlined in the above article, Coca-Cola was not losing sales to a competing soda, it was merely some die-hard fans celebrating a product they liked. In that situation, suing them would have damaged the company's goodwill and potentially reduced sales. On the other hand, if a competitor is mimicking your trade dress or mark and selling a confusingly similar product, taking legal action will probably not garner as much negative publicity.