To preserve everybody from spoilers, I will only post the beginning. Links will be available if you want to read the full reviews.
The Hollywood Reporter : Stoners take a bloodbath in American Ultra, a genre mash that’s mildly amusing until it can’t think of anything else to do besides flop around in the deep end of conspicuous gore. Taking a vacation from more serious projects by playing a couple of lethargic, ambition-free tokers who suddenly find themselves in the middle of a Joe Carnahan movie, Jesse Eisenberg and Kristen Stewart smoke and say “like” and the F-word a lot and eventually kick plenty of butt in a way that looks to cook up a couple of weeks of buzzy late-summer business with good-times-seeking young audiences…
MovieManzel : The Summertime High That No One Saw Coming.
Mike Howell (Jesse Eisenberg) is an anxiety-ridden Cash & Carry connivence store clerk that sits around drawing his Apollo Ape comic while getting stoned every chance he gets. His girlfriend Phoebe (Kristen Stewart) takes care of him and loves him even with his flaws. Her only wish, however, is that some day Mike can beat his anxiety so that they can leave their small hometown. One day, Vanessa Lasseter (Connie Britton) walks into the Cash & Carry and informs Mike that the CIA is coming to destroy him. Confused by this interaction, Mike ignores the warning until two police offers show up and try to kill Mike. This attack activates something inside Mike that turns the once anxiety-ridden stoner into a killing machine.
ScreenCrush : A late August release date typically means one of two things: A movie is terrible or a movie is weird — and therefore difficult to market. American Ultra is a classic example of the latter. The trailers and posters mostly sell it as a wacky stoner action comedy, but drug humor actually plays a fairly small part in the film, which cycles through scenes of bloody horror, government conspiracy, and sincere relationship melodrama. Audiences might go into American Ultra expecting The Bourne Identity meets Half Baked. They’re going to find something much stranger and more interesting (with a bunch of First Blood, a dash of The Cabin in the Woods, and even a smidge of rom-com tossed in the mix).
Entertainment Weekly : Mike Howell (Jesse Eisenberg) is pretty much a loser: a feckless small-town stoner who spends his days drawing homemade comics behind the register at a near-deserted Cash ‘n’ Carry and his nights getting high and burning omelets in his dingy bungalow. But he has a beautiful girlfriend named Phoebe (Kristen Stewart) who loves him despite his dim prospects, and even forgives him when a crippling panic attack prevents him from getting on an airplane bound for their dream vacation in Hawaii.
RamaScreen : AMERICAN ULTRA rocks my socks off!
In the words of “SouthPark”s Towelie, ‘You wanna get high?’ Yes, I do want to get high on #KristenStewart and #JesseEisenberg, cinema’s most awesome couple this year in #AmericanUltra a movie that rocks my socks off. No better way to describe it than if Jason Bourne was a stoner.