The idea behind Devil is great, five strangers trapped in an elevator and bad stuff starts to happen. The main problem is the element of the title. In my opinion, if they left the devil out of this film it would be a superb thriller, instead of being a supernatural flick that's only slightly above average.
Detective Bowden (Chris Messina) is called to investigate a suicide in downtown Philadelphia. Meanwhile, five seemingly random strangers get trapped in the elevator of a nearby skyscraper. Bowden is called to assist in the matter after one of the trapped civilians dies. From the security office Detective Bowden oversees the incident that's unfolding in the elevator.
Like many supernatural thrillers, the protagonist is skeptical while there is a very superstitious side character constantly warning him of the dangers of the devil. The functionality of this character is useless until near the end, but this is a good thing. The side plots behind each character's motives to kill the other members of the elevator are good, for example one woman recently divorced a rich man, and one of the men has a history of violent offences. Unfortunately none of these motives pan out as the end result of the film is that the devil is involved.
Another flaw with this film is that it brags to come "From the mind of M. Night Shyamalan". No offence to him as he has created many brilliant films, but you need to know when your time is up. Luckily, Devil has little to do with Shyamalan and the film remains differentiated from his recent flops (The Last Airbender and The Happening to name a few).
Overall Devil is a good thriller, it maintains a good plot until the end, and it's short enough that it doesn't get boring in the elevator. Devil isn't really scary, at least it won't be for most people, but it's still a good thriller. I just wish that the film explored the subplots further and perhaps expanded upon one of the other areas opposed to bringing the devil into the mix.