I also walked out on Tomb Raider (the first one). I don't know anyone else who thought that movie was a complete and utter bore. Even the eye candy (Angelina, duh) wasn't interesting enough to keep me there.
I know I've walked out of a movie before, but I can't remember which one. But hands down, the worst movie I've ever seen is XX/XY. My ex and I turned it off like half way through, just before it was enough to make us shoot ourselves in the head.