Shoot ’em Up is a movie that is in all ways gratuitous. Gratuitous sex (only one scene, but it’s really gratuitous). Gratuitous gore (lots). Gratuitous violence (lots doesn’t cover it; “constant” comes closer). No real character motivation, plot, or relationship with physics unless you’re willing to take your disbelief out back and shoot it.
I had mixed emotions about this movie. On the one hand, really over-the-top violence is sometimes hilarious (Kung Fu Hustle is possibly the greatest action movie ever). On the other hand, I’m often very uncomfortable with realistic violence (trying to watch Braveheart was a disaster). I probably won’t watch Shoot ’em Up again because it was a little too violent/graphic for me, but I did laugh most of the way through. Your mileage will vary; rent first.
Objectively, it’s a really bad movie. But bad intentionally, and pretty hilariously.