Does sunscreen work? I've personally heard an answer in the negative dozens of times.
This comes from a study some researchers did a few years back comparing mortality between sunscreen-using and non-using populations, and found that sunscreen didn't provide any benefit. It was one of those results perfectly suited for endless repetition.
However, as Aaron Carroll mentions below, this result is not very strong. On the contrary, if you put it on right, sunscreen probably does prevent skin cancer, and is definitely worth using. --Ryan Cooper