If you haven't seen any of the trailers, the movie is being sold as "Lucy (aka Scarlett Johansson) gets used as drug mule, the drugs leak, she gets magical super-powers, then has to get away from/beat up the drug dealers."
I'm sure almost everyone has heard the "humans use only 10% of their brains" bit that's prominently featured in the trailers. I hope that everyone reading this knows that's an urban legend that was refuted a long time ago.
I'm convinced that when this movie was first conceived Luc Besson, the writer and director, said "That 10% of the brain thing may be wrong, but i don't think it's wrong _enough_. I believe we can make it wronger!"
As a very specific example, do you know how dolphins are able to do echolocation? During the lecture by Morgan Freeman (which is interspersed with the first third(?) of the movie) it is explained that dolphins are the only animals that use more of their brain than humans do. It is because they use 20% of their brain that they are able to use sonar. The conclusion? If humans learned to use 20% of their brain then we'd be able to use sonar too! (Lucy doesn't actually get to use sonar when she hits 20%, because that would be entirely too mundane and not nearly insane enough.) (Also, how do bats use sonar since they don't use 20% of their brain like dolphins? Who cares!)
So it's a bit of a spoiler, but i think it's fair to warn people that if you're expecting a lot of life and death fights between Lucy and the drug dealers, don't. This movie is about something entirely different. There is fighting and conflict but... well, spoilers.
But if you're okay with a totally insane action art flick it's pretty good. (Though i think i'd still recommend Snowpiercer over it if you're okay with gore and a somewhat nihilistic ending.)