S/he who stays in the rear, takes it in the rear.
SHOOT 'EM UP amounts to a delicious debauchery in violence and sex. Mostly violent, mostly brutal, completely facetious, the key is to never take this movie seriously and just have fun. If you're offended by violence for violence's sake, this one is not for you. Some of the ridiculous stunts our protagonist Mr. Smith manages makes it difficult to suspend disbelief, but it's still fun if you're laughing with the movie instead of trying to take it seriously. There's scenes where he's handling a baby like a football in one arm and somersaulting in the air gunning down villains from the other hand. There's another scene where he's having sex pumping away and shooting the baddies from around the woman's shoulder. It qualifies as a fulsome caricature in violence and it's pretty graphic as well. Basically it's fun.
The story is fairly simple enough. Clive Owen plays "Mr. Smith," a guy who rescues a pregnant woman chased by some goons. Smith's arch nemesis is Hertz, played by Paul Giamatti. Paul Giammati and Clive Owen deliver convincing performances in spite of a dearth of acting opportunities. Clive Owen is his no-nonsense hardcore self while Paul Giamatti's Hertz, a former psychological profiler, chases Smith. Smith digs deeper behind why the goons wanted to kill the pregnant woman, and a plot using the bone marrow of illegitimate babies surfaces. All of it, however, takes a firm backseat to the movie's central intent: gratuitous violence and action.
Want to know the difference between a luxury car and a porcupine? The car has a prick on the inside...
The movie is pure, unadulterated fun. It's funny too.