The most recent one I can think of is Scott Pilgrim vs. the World
I saw a trailer for it, looked like a crappy romantic comedy, so I wasn't going to bother. Then my friend asked me if I'd see with her. I figured what the hell, went and saw it, loved it, bought the graphic novels. Now I'm a big fan. Sometimes, you are surprised!
On the other hand, one time I was invited to watch Dragonfly
with my family, and I knew nothing about it so I naively agreed. Kevin Costner could die in agony a thousand times and he will still not have paid for the thousands of hours of my life that "movie" took from me.