There are some action movies that feature a rich storyline where rampant violence is contextually appropriate. But most are riddled with half-baked plots that are a weak excuse for a series of action set pieces. There's nothing inherently wrong with that, unless they take that plot seriously, which for some reason almost all of them seem to do. Thankfully some action movies realize that the whole genre is ridiculous and leave the plot behind entirely. Of course your enjoyment of these is gated by whether you can consider excessive violence funny.
Shoot 'Em Up is bound to be a polarizing movie. It's intentionally shallow and gratuitous. There's a plot, I guess, but it only kicks in after you're already knee deep in over the top gunfight choreography. And if you think a movie of guns and one-liners sounds awesome, then you will enjoy Shoot 'Em Up. If you don't, chances are you'll be mortally offended. There's not much more I can write to sway you one way or the other, because all of the quality of this movie is visual and/or contextual. I guess I could describe it as a cousin of Hot Fuzz but darker and less obviously funny, if that helps. Or you can just go watch a trailer. Or I could just sum it up with one statement: "multiple vegetable-related fatalities."
I guess what I'm trying to say is that if you're jaded and desensitized like me, Shoot 'Em Up is freaking awesome.