A while ago, I wrote a post about why competition is good for your business: it stimulates you to innovate and never relax (Michel summarizes it greatly here). I also did another post on why you need to monitor competition all the time, and mentionned for example this tool, which is not sufficient as such, and you need a number of others that we use all the time (pricing, service offering, product mix, etc.).
I really believe, as an entrepreneur, that your business is good because you provide value for money to your customers, and hold them in respect. No cheating in my workplace. No badmouthing.
2 years ago, one of our employees inserted one of our competitor's name in the meta-tags of our pages. Once we discovered that, we immediately removed that, and that employee, among other reasons, is no longer with us.
I don't mind my competitors copying my service. Really. Loic once wrote that Ideas don't matter, only execution matters, and I agree with him. Now, if you develop something fancy that brings you competitive advantage, you should simply patent it, and do all the legwork to protect your investment.
Now, what I really do not like, is when my competitors start playing foul. Click on the picture to the left. You all know how advertising on Google works: you buy a keyword (sometimes you have to bid really high), and you pray (actually you do some maths) that your ROI on that is good. Now when your competitors start buying your brand as a keyword, that is called not only cheating, but brand infringement. Google France has been taken to court recently, and has lost its case to 'la Bourse des vols' and to 'LVMH' for selling brands as keywords. Our brand, glowria, is regularly registered at INPI.
My competitor is going to be talking a lot to my lawyers as well.