Just the other day I received what I felt was abominable service. I left 10% and noted on the check it was not a mistake. I wrote that the service was terrible. I left 10% rather than nothing at all because in more expensive restaurants there are several people who rely on the tips left by customers. I hate to make them suffer for the one waiter, but I make darn sure management knows I'm unhappy.
On the other hand, in less expensive shops or cafes when I receive service above and beyond the call I often leave ridiculous percentages. (50% to 100%) It often amounts to just a few extra dollars, but in the heart of the server it often makes their day/week/month.