Have you ever been interested in or involved with someone who was interested in S&M?

A friend of mine recently estimated that interest in S&M is way more common than most people think. I would say that in my dating history of roughly 8 years I have only encountered two people who were into it (spaced apart by 3 years), so it seems relatively uncommon to me.

How common has it been in your experience? Is anyone out there into it? If so, why? How did you get interested in it? Can you go through an S&M phase and then lose interest in it? Is the general consensus that this is a normal interest, or is it an indicator that someone might be emotionally unhealthy? Is anyone willing to just generally talk about their experiences with it?