I remember a book I really enjoyed, in which the 'F' word appeared maybe three times. It was an Anne McCaffrey hardcover I had called Dragonsdawn and it was one of her Dragonriders of Pern books.
The fact that the word appeared as late in the book as it did, as well as so few times--all in one chapter, as I recall--gave an effect of shock and surprise, but at the same time, it was not a bad effect. The minimal use of just the right words really made the book come to life more to me than if it had been used throughout the book.
So I'm of the opinion that moderation is a good thing where profanity is concerned. It increases, at least by my own estimation, the enjoyment of a work and doesn't turn me off to a piece quite as quickly, because the profanity is strategically placed in the work instead of strewn throughout for no reason.