It may sound boring that an entire movie is set within an elevator but, perhaps luckily, there are just as many (if not more) scenes out-with the elevator, mainly of outsiders trying to help and just try to figure out what's going on as the incidents in the elevator get more horrific as time goes on.
I was afraid going in that Devil would resort to cheap gore tactics or out of ill-timed jump scares but director John Erick Dowdle (of the U.S. [REC] remake, Quarantine) really knows how to handle the horror. The use of the lights cutting and out is quite brilliant and the fact that all the actors are convincingly scared doesn't hurt the film's believability. There are a few moments here and there that aren't for the faint-hearted but for the most part the terror is classy (if that word can ever be used to describe a film like this) rather than vulgar or offensive.
On top of Devil being a genuinely entertaining horror-thriller, in the end it also has something to say about redemption and being truly sorry for anything wrong you may have done in our life. The way the morals are presented might seem a tad tagged on - a certain clumsiness of how it jams those scenes in there take away from them somewhat - but nonetheless they're still effective.
Devil won't go down in movie history as one of the classic horror-thrillers but for what it is its solid entertainment through and through. It takes you to some places you might expect and does all it can to surprise with its many twists and turns (whether you guess the big twist will depend on how much attention your paying). This is the best piece of cinema Shyamalan has been associated with in years.