First a couple of caveats - I'm running this game on a High-ish spec PC and I am very much a fan of traditional survival horror games.
The Evil Within is a game for survival horror fans and it really doesn't make any excuses in that regard, people who didn't like resident evil 1-4 or the Silent Hill games are unlikely to find a gateway into the genre through this game. The difficulty is punishing, every enemy is a threat and your brain is almost always your best tool.
Atmosphere is the game's real strong point. A mixture of disturbing sounds and visuals ensures that the only time you feel safe is when you can hear the familiar theme of the mirror world. (Not actual name just my nickname for it.) This is a safe-house type area accessed through cracked reflective surfaces (usually mirrors) where the player can save and upgrade various aspects of their character and weapons however even this location feels distinctly off and while you feel safe from enemies you always wonder when this safety is going to collapse. This is also your primary location for putting together the pieces of the game's story. Entrances to this location are usually accompanied by extracts from the main character's journal, the newspaper stand provides a steady stream of articles and missing persons posters make it seem like the world is being slowly swallowed up. The game's story itself is the kind that will have you theorizing constantly about the nature of the game's world and each character's role in the story. A few things are spelled out plainly while the rest is left to the player to draw their own conclusions.
The gameplay reflects the visuals, keeping things disturbing and unerving for most part. You can't move and shoot which limits your options to stand and fight or flee but the game backs this up with a decent stealth system that allows the player to perform sneak kills, hide and distract enemies. The player can use matches to burn fallen or knocked down enemies and this actually has some combat use since nearby enemies will also be set alight whether knocked down or standing. For action game fans these controls will feel restrictive but that's very much their purpose creating real tension in every encounter.
The games encounter's themselves are mostly excellent. Of particular note is a section with invisible enemies who can only be located by listening and watching for movement in the various bits of physics enabled clutter which cover the area. Many of the boss encounters also make great use of build-up and usually require more in depth thinking to defeat than simply shooting it till it's dead.
However the game does have its miss-steps which cause frustration and these are mostly to do with insta-kill moves/enemies. The games lead antagonist appears in several sections and will kill you with a single touch, avoiding him is not all that hard but it often leads to dull activities such as hiding till the encounter is over or running round in circles keeping him in view to prevent him teleporting in front of you. An extreme source of frustration is a boss with a long first phase followed by a second stage which has an insta-kill move which it attempts constantly. The games greatest miss-step comes in the form of three chapters in the mid-late game which seem to completely ditch survival horror in favour of action set pieces including a turret section which don't gel well with the game's tone or mechanics. Fortunately the section is short lived and we are soon back to claustrophobic corridors and scary lighting.
The game's only other real issue is its protagonist. The game sets up his back story and relationships but we never really see these reflected in actual scenes. For example the game repeatedly highlights the character's drinking problem in journal entries and some dialogue yet we never actually see him drinking or even craving a drink. He is also very inconsistent on his understanding of the gameworld wondering whether or not it is real even after he has been given a specific answer to that question which makes him just seem incredibly dumb.
Some have complained about the inconsistency of auto saves and repeatedly replaying certain sections however I generally consider this to be part of the genre as a whole and having auto-saves at all means its by no means as punishing as resident evil one, with its limited typewriter reels.
On a decent spec PC the game looks great and runs well. I have heard complaints of low res and bad performance on other systems but all that really serves to do is highlight how far behind the "next-gen" consoles are (apologies for the PC elitism). The back bars aspect ratio is a little odd but I understand the thinking behind it which was to stop the player seeing their feet adding to the tension because the player doesn't always know what they're stepping in. If you don't like it the PC version does include console commands that can be used to turn them off. There is also a console command for unlocking the game to 60fps however full support for this is said to be coming in a patch further down the line so there may be some issues at present.
In conclusion then this game is pretty much a must for survival horror fans but others should probably be a little more cautious. If you like punishing difficulty and using your wits more than your weapon you may well enjoy the game but you will have to be prepared to tolerate the quirks of the genre as well as the few miss-steps that knock down my final score for the game to an 8/10.