I have strong mixed feelings about this movie. Lets have a quick look at what those mixed reactions are

Whats Good?
Great fusion of genres - mutations, time travel, generic sci-fi and superhero. Popular casting of actors who the audience love to see. Screenplay that holds your attention for most of the runtime. Interesting concept bringing back memories of the Terminator. A terrific performance by Pierce Gagnon as an emotionally volatile kid with a powerful ability. Most of all, it has some powerful moments that touch your heart.

Whats Bad?
Introduction with a flat storyline that doesn't raise a pulse until the young kid shows up who is the heart and soul of the movie. The gimmick to make Levitt look like Bruce Willis was a farce beyond repair. And Bruce Willis looked so out of place with both his mannerisms and lines until he was given a gun and allowed to go bombastic. It suffers the same fate as most other time travel based stories do - impossible flawed conclusion. Future self meeting the current man can lead to very interesting conversation, this moment was ruined with just more bang bang.

Concept:
In the future when bodies are impossible to dispose off, criminal gangs make use of the already invented time travel technology to send people back 30 years who they want to get rid off. A 'Looper' waits for the person to arrive, kills them and dispose off the body. A Looper gets well paid but there is a catch. At some point a future self of the Looper will be sent back who is to be killed the same way as any other but comes with a big payday and a retirement. Letting a kill get away is a death warrant.

Story:
Joe (Joseph Gordon-Levitt) is a Looper. When his future self (Bruce Willis) comes back, a moment's hesitation allows older Joe to get the best of him. After a few tussles, tracking down etc., older Joe tells the story about a man calling himself The Rainmaker who spells deadly terror in the future taking down all the criminal syndicates and closing down all the loops. He has narrowed down on the Rainmaker's identity who will now be just a kid (Pierce Gagnon) son of Sara (Emily Blunt) and wants to take him down.

Movies involving time travel should have some basic elements right for them to work since the concept by itself is flawed anyway. Either the plot has to be extremely complex to take the viewer's mind of the general logics that surrounds it, or overcome it with downright entertainment. Looper falls short on both directions. Levitt's makeup to look like Bruce Willis makes him incapable of conveying the right expressions. No great innovation or awe in the special effects department either. Bruce Willis first few dialogues with Levitt are laughable and can be mistaken for The Expendables.
But with some smart screenplay, charismatic Emily Blunt and a performance by Pierce Gagnon that could blow you away.

Manages to stay afloat even after treading on prohibited territory abandoning the most desirable path.