8 years ago I had a friend and his girlfriend visiting, and they were on MC. They raved about it, of course. I still thought they were crazy, but he had Crohn's disease and was trying all sorts of different therapies, so I didn't harass them too much.
I finally decided to try it in 2004 because I felt toxic from years of partying, my damaged joints hurt, and I was carrying about 10 extra lbs.
Also, I like unusual experiences.
I researched it thoroughly before starting, which really helped me cope with the various sensations. Still, that first Master-Cleanse was very difficult, I had a lot of detoxing to do and I felt really terrible much of the time.