Etiquette when talking to a researcher whose work you do not think highly of
Be polite. There's nothing to be gained by being a jerk. It's funny in the movies, but do people really say things like that in real life?
Be diplomatic. State initially that you do not fully understand the approach or the motivation, and then after some discussion state that you do not fully agree with the approach taken.
Be succinct. Point out clearly what you see as being the problem with the paper.
Be constructive. Indicate places where you think the paper/research could be improved, with concrete suggestions of how you would make the improvements. 


Alternatively,


Avoid the topic. Talk about the football scores instead. What about those Bears?