In the session I DMed last week, I had the party fight their mirror images. It was a tough and long fight and it really came down to the luck of the roll. For Belen, luck was certainly against him in that fight. The mirror image of his halfling wizard was just kicking his PC's butt! In three rounds, his PC was on the ground, bleeding out. Belen's first reaction was "Help! Somebody heal my character!" After a moment's thought, though, he got this really wicked look in his eye.
"You know," he said, "I think this is the perfect time for the mirror mage to cast fireball." They were fighting in a very small room which had no visible means of escape. Fireball most definitely would have killed his character.
Lucky for me, the mirror mage was killed before his initiative came around again. But, it made me realize that I had a player who had definitely lost interest in his character.