All the actors were outstanding in this movie.
Jack Nicholson was awesome as the devil , Mark Wahlberg was awesome as the angry cop , Leo was tremendous as the rat , and Matt Damon was ( I can never see him in a film without thinking about Team America World Police MATT DAMON!) good as the other rat.
The ending feels like the writer saying "how quickly can we get rid of all these characters and end the movie."
Although the ending was pleasantly shocking in some ways , I felt quite ripped off by it. It might grow on me later , but I'd have to watch it again.
So I give it a 7.7/10. If the ending matched the rest of the movie it would be 9.5.