I've seen this called a cross between The War of the Roses and True Lies, and that isn't that far off the mark. It isn't quite as dark and intelligent as the former, and it is not as adventurous as the latter, but what it is is an absolute blast from start to finish. It is a perfect example of what fun cinema is, it doesn't aim too high, rather it banks on the chemistry of it's leads to bring plausibility to completely implausible situations.
The story is simple enough to outline without giving anything away, as the plot is not what this movie is about. John and Jane Smith are married, they each lead a secret life as assassins. When they leave to go to work they are not going to the office that they claim to be, rather they are going on globe trotting missions to find their targets.... and kill them. At night, or after a "business trip", they return home to each other and their mundane lives. That is until they are both sent on missions with the same target, then the secret is out and all hell breaks loose. The marriage dissolves and they start targeting each other. They slowly realize they do actually love each other, get back together and start fighting everybody else. That's pretty much it.
There are a couple of factors which make this movie rise above the typical bland action/comedy. First and foremost are the fun performances from the leads Brad Pitt and Angelina Jolie. Another are the supporting performance from Vince Vaughn and the faceless hordes. Thirdly the script from Simon Kinberg. And finally the direction from Doug Liman.
First the leads. Pitt and Jolie put on a sheer display of star power and have great chemistry on the screen. Regardless of the off camera stories, when they are on-screen together it is fantastic. the timing is impeccable, their ability to ground the free flying antics and make it believable is a great feat. Jolie looks great here, and Pitt pulls off the nonchalant suave like no other, almost a continuation of his Ocean's 11/12 character. When two actors have this chemistry, it is hard to describe, but I will say that I wouldn't mind seeing them work together again in the future. Not necessarily a sequel, but something, perhaps a romantic comedy that pays homage to old school films from the 50's and 60's?