If you're looking for some mindless action and excellent snark, this is the movie for you.
I can find fault with the pacing and the paper-thin plot, but the reason for the movie is obvious: Ryan Reynolds and Samuel L. Jackson trading quips and being generally hilarious. When they do that, it's definitely worth the time.
The general idea: a dictator (Gary Oldman) is on trial, but all of the witnesses against him are either dead or discredited by lack of hard evidence. The last chance to convict him lies with notorious hitman Darius Kincaid (Samuel L. Jackson), but Interpol has been compromised so he needs someone to get him safely to the trial. Enter disgraced protection agent Michael Bryce (Ryan Reynolds).