I like Lowep's post best so far.
I believe the best sharpening method begins at time of capture. Before any sharpening is done, I blow my images up to at least 100 percent, and if they don't hold up, I trash them. While the saying, "garbabe in-garbage out" is an exaggeration, there is some truth to it.
Like with other aspects of PP, the less you do the better, because any adjustment you make in PP doesn't just change the target aspect, but many other aspects throught the spectrum. Introduction of artifacts is the most common problem with sharpening. One of the most common reasons for rejection with stock agencies is "oversharpening," and it doesn't take much to bring that verdict. They have good reasons for such strigent requirements, but they, too, are based upon output IQ.