I, too, will go well above the 20% rule, and go as far as matching the check when the service has been exceptional.
At a recent meal, I had invited several friends (and their children) to join us for lunch (and our children). Needless to say, with 10 adults and 7 kids, it was a real fun time for the waiter. But, he was a sport! When the kids spilled drinks, he was right there with refills, stickers & pre-meal 'snacks'. The works. He brought their food out early and even re-plated a few dishes that were too hot for small children. I left him 80% of the check and wrote a letter to the restaurant. I got a call the day they received the letter and told me that they had awarded this particular waiter a 'bonus' and thanked me for taking the time to write. As a member of the trade, I certainly appreciate when someone takes the time to thank me for a particualarly good meal, so I recipricate whenever possible. However, if the service is poor, so is the tip. If it is poor enough, so is the letter. After all, you are purchasing a meal; a meal being the food, the atmosphere, the service, the cleanliness, the whole package. If the package is defective, it needs to be discussed and repaired.