Done well, such as in Christmas Vacation it can be effective, but yes, if done poorly, adding obscenity upon obscenity is just embarrassing. I think what makes Clark Griswold's delivery so effective, and MeanJoePhaser is correct, it is all about delivery is that it doesn't just roll off the tongue. I know I get tongue-tied and at a loss for words when I'm very upset, so it would be natural to stumble and concoct odd expressions when you are that upset about something. The ones that easily roll off the tongue sound practiced, and in my opinion fail for the most part.
The exception that fits that rule would be Sgt. Hartman as perfectly portrayed by R. Lee Ermey, because he does come across as a guy who seems to incorporate stringing together insults, obscenities, curse words, etc. to degrade his soldiers. I see him as so practiced he can string together the most vulgar things and make them sound equally off-the-cuff as if you pissed him off to the point where he has no choice to say them, as well as well-practiced as if he sat up nights dreaming of insults to use on you, and you in particular.
