I like Eisenberg and Nivola, so I was primed to enjoy this, but what a letdown. It starts off with a decent droll tenor, even if the humor is pretty lame and the story very simplistic (fraidity-cat nerd gets mugged, then gets empowered in karate class). But rather than evolving into something more interesting, the film just gets mean-spirited without truly going "dark," with improbable developments and insufficient depth to pull them off. Characters who turn out to be sorta evil remain cartoonish, so there's no punch to the revelations.
This is neither a farce or a "black comedy," just some tepid compromise between, and it wastes some very good actors. You've seen Eisenberg playing a dweeb before--he's certainly good at it, but this movie doesn't bring out anything new--while Nivola, who's normally terrific, is neither very funny or sufficiently threatening in a role that calls for both. Likewise Imogen Poots and David Zellner get one-note roles. I don't know what this movie was trying for, but it didn't work for me. I would have rated it lower (because I really did wind up disliking it), but in technical and acting terms the filmmaking was competent enough to give it a medium grade.