Like most things, I think it depends greatly on your specific situation. It might work on some sites and it won't on others. If you make more money than the sales, then obviously it's worth it. I tried doing it for a while, and for the tiny extra amount I made each month, I decided it wasn't worth losing customers or distracting them. But my sales far exceed anything that I would make with ads. Especially now, since the whole AdSense thing seems to be sinking a lot. I don't even see relevant ads on my other sites anymore.