I'm about halfway through, and my consensus so far is that it's the sequel/successor to Resident Evil 4 that I wanted out of Resident Evil 5 in overall design philosophy and vision, drawing very heavily from core gameplay systems, visual concepts, pacing, imagery, and so on, and this is the game's strength. It does this while introducing its own systems and ideas, which do a go a long way towards avoiding too many gameplay similarities, while also helping forge The Evil Within's own identity.
That being said, while it might seem like a successor to Resident Evil 4 it does, quite often, feel like it's living in the latter game's shadow. Original ideas are introduced and abandoned too quickly or not expanded upon as much as they could have been (pending the latter half of the game). The parts strongest in similarity to Resident Evil 4 at times feel almost like carbon copies, only worse, largely in part due to the lack of originality. This includes gameplay set pieces, level design, and locations.
In fact, so frequently does The Evil Within draw from Resident Evil 4 and other Mikami titles that, given so many cut/paste settings, I'd almost credit it as some weird meta commentary on Shinji Mikami's brain and the shit he keeps thinking of. "The Evil Within" is Shinji Mikami's head and his creations.
In the long run this lack of originality hurts. I think the game looks gorgeous and has fantastic art, but when I enter an area and, no matter how pretty or well visualised it is, when the first thing I naturally think of is "Yep, Resident Evil 4" this is not a good thing. It happens too frequently, like the designers pushed "Resident Evil 4 successor" way too far to the point of just re-imagining chunks of the game. Only given how fucking fantastic Resident Evil 4 is from top to bottom, these re-imagined chunks just don't play as well.
There's been a handful of parts that I think are legitimately miss-directed, specifically parts where you have to do or play exactly what the designers want to forward the set piece, narrative, or whatever. The game is rarely scary, even though I love the monster and location art, and only occasionally tense. And even though it does seem trapped under the blanket of Resident Evil 4, most of the game is still legitimately good. Outside of some dips so far, the pacing is sublime and varied (in an overall package sense, not originality), and the core gameplay systems are well implemented and satisfying to use.
Thus, as a whole at about the halfway point, this is a really good action/stealth horror themed video game. But it's no survival horror, and no Resident Evil 4.
