Into the Dead: Our Darkest Days, besides being a needlessly long title, is a curious case of what happens when you mix This War of Mine with zombies. But even as someone that loves both of those things, I must admit I was genuinely surprised by how much Our Darkest Days ended up gripping me.
A lot of it comes down to the perfectly executed atmosphere of stress and hopelessness - where every single excursion outside your safe zone feels like you're a hair's width away from death as you sneak through hordes of gurgling, twitching, and utterly disgusting zombies. It's all incredibly stressful - but the good kind of stressful!
Video version of this recommendation (~12 minutes)
Surviving the zombie apocalypse
When it comes to the actual gameplay, Our Darkest Days has its split into two distinct parts: base management where you take care of your survivors, build up your shelter and craft various items; and the expeditions into zombie-infested areas where you search for resources, additional survivors or even a way to escape this whole nightmare.
The base management is fairly similar to This War of Mine. Each day is split into two phases, day and night, during which you'll be able to allocate your survivors to specific tasks. You can have someone spend the day cooking, another person planting crops in your rooftop garden, and a third one mass-producing weapons for future expeditions. On its own, the system is very simple and easy to understand.
All of the difficulty comes from the fact that you never have enough resources or time to get everything done. Planting crops is great, but those things take a while to grow and your people are starving right now, so can you really spare the veggies? Upgrading your workbench requires an absolute ton of metal, and while it would be handy, can you really say 'goodbye' to five or so knives in order to make it happen? And what about your best fighter? How often are you willing to risk their life - and sanity - for the benefit of the team? What happens if they crack - or lord forbid - die? Are you just boned then?
The obvious solution is to bring in more survivors to bolster your numbers, but more hands means more mouths to feed and more potential problems since each survivor comes with their own pros and cons. One survivor might be incredibly good at cooking but terrible at physical actives, while another might be feisty in a battle yet mentally unstable outside of it. There's a tricky balance to be struck here, and even though I think I know what I'm doing, it's still never comfortable. It always feels like the group is one bad looting run away from total collapse, which really helps sell the idea that these are the end times. The whole thing is stressful, but I'd say it's the good kind of stressful - the kind that keeps you sharp and immersed into the world.

Risking it all for soup
So how do you get new stuff? Well, each safe house has a certain radius around it that you can pillage for resources. The bigger and more infested the location, the more items you'll likely get out of it, though obviously the bigger the risk. And if you ever exhaust all of the nearby locations, you'll have to pack up your bags and move the group elsewhere. That's probably a good thing to do regardless as the longer you camp at any given place, the more harrowing the zombie presence becomes.
Once you've selected a location you'll be prompted to give an unlucky survivor of your choice an assortment of weapons and tools to brave the challenges that await them. The more you give them the greater the chance of success they'll have, but if their hands are full of knives and frying pans they won't exactly be able to bring much stuff back. Despite this, I still tend to over-prepare as it's a lot easier to just send a worse survivor back later to pick up the random knickknacks than it is to have your star athlete free and in peak condition.
From what I can tell, each of these locations - and there is a lot of them - is handcrafted, though the actual zombie spawn locations are semi-randomized. So while you can go through the same sort of buildings in two separate runs, chances are good they'll still be able to surprise you. So in order to give yourself the best possible chance at success, you'll need to disassemble each location as stealthily and methodically as possible.

If you can, do try to be sneaky
You can fight, but...
The zombies may be dumb, but there is loads of them out there - though perhaps not in a form you might expect. You see, Our Darkest Days plays like a 2.5D game where you are mostly locked down to a single plane, and so any zombie in the foreground and background will not see or hear you... until you mess up and alert them. Once that happens your best bet is to run because while combat is possible, it's intentionally awkward in order to discourage you from trying to roleplay The Walking Dead crew.
Even a single zombie is a threat, while anything past that is likely going to result in you taking some serious damage, especially if they start to surround you. Compound this with the fact that weapons have limited durability and will break easily if you're using them to bludgeon skull after skull, and you've got yourself a pretty good incentive to stay quiet. That is, unless you have a working gun, in which case you can go on a rampage... all the way until you run out of ammo and are stuck in a hell of your own creation.
So while you can fight, try to avoid it as much as you can because Our Darkest Days is primarily about being sneaky and clever. Why waste a perfectly good baseball bat smashing through a massive group when you can instead use a pair of rusty scissors to dispatch a patrolling zombie from behind and ensure you're never discovered. And this right here is what managed to get me hooked onto Our Darkest Days.

It's really easy to get overwhelmed if you're careless
Be clever or join the ravenous hordes
There is just something primal and terrifying about sneaking through a decrepit, creaking house like a little mouse - constantly on edge from the endless gurgling groans around you, yet never sure which one is actually active and which are just caught up in their own little world of misery. That's the interesting thing about Our Darkest Days' zombies. They're not undead as far as I can tell. They feel more like sick people whose bodies and brains have been hijacked and twisted against their will, so when they're not burning with murderous rage many of them are just crying, trying to stick to old habits or are simply slumped over like a discarded toy.
All of this gives them a very unique vibe and somehow makes them feel both pathetic and horrifying at the same time. Peeking through a door at one twitching and contorting itself as it attempts to walk around the remnants of its former room is genuinely unsettling - even more so when you realize you're going to have to open the door and get in there if you want to have dinner tonight!

The zombies frequently look sad and dejected... right until they try to maul you
Naturally, things don't remain that simple for very long. Once you're out of the suburbs the locations you can visit start becoming more and more complex - not to mention massive. Ordinary homes get replaced with multi-story buildings with barricaded rooms, locked doors, caved in walls and floors, as well as various traps that may or may not be obvious. So even though there are no special zombies that hurl acid or suplex trains for fun, the difficulty still kept increasing at a steady pace all throughout my campaigns.
A good thing too, because I found the challenge of conquering these big buildings to be quite exhilarating. Incredibly stressful to be sure, but also exhilarating! It's never as straightforward as clearing things room by room. You frequently have to make detours, utilize the environment to reach different floors, intentionally aggro zombies to corral them to less troublesome areas, and pretty much try every trick in the book in order to make sure you don't waste your entire armory on a single expedition. And it likely won't just be a single expedition, because some of these places are so big you'll need a couple of days to properly strip them of any valuables.

Be very, very quiet!
Dread, despair and a shred of hope
This is where I need to bring up the atmosphere once again, because that is what makes all of these large-scale missions so tense and memorable. Doubly so if you decide to tackle them at night in order to be efficient with your time. When you're sneaking through hell guided only by a tiny flashlight, the endless groaning and mumbling of the zombies mixing with the sounds of distant footsteps somehow becomes even more unsettling. I can't even explain to you how awful it is to be walking through an apartment while hearing activity all around you, yet being completely unaware of where it's actually coming from. Yet that's also what keeps me coming back to Our Darkest Days. Sometimes you just need a little bit of controlled 'awful' in your life, or that could just be me being weird!
Once you get tired of the apocalypse, or once the resources begin to dry up, you can also start thinking about going through with an escape plan. These are essentially quests you discover piecemeal while exploring the levels. Maybe you'll hear about a helicopter pilot being stranded somewhere, or a group smuggling people out of the city through the sewers. Whatever the case may be, following through with one of these plans will eventually see your group achieve something that is quite rare in zombie media - a happy ending! The ending was obviously unavailable in the demo so I can't comment on that, but what I can say is that I do like it when survival games give you some kind of goal to work towards that isn't just "don't get eaten today".
And finally, I just want to briefly touch upon the visuals. They're not particularly high fidelity, though funnily enough that actually works in Our Darkest Days favor since the grungy, slightly muddy textures really help push the idea that civilization as we know it has collapsed. So while I'd like to see foliage spruced up a bit as it looks distractingly bare-bones compared to everything else, overall I'm perfectly pleased with the visuals. They might not be anything spectacular, but they are highly immersive, and for a zombie survival game that's really all that matters.

Closing thoughts
You know what the best thing is? Into the Dead: Our Darkest Days has only just launched into Early Access, so everything I've shown you is likely to get much better and more polished as time goes on.
So if any of this looks like it's right up your alley, I'd highly recommend keeping an eye Our Darkest Days' progress, as well as giving the free demo a try on Steam. It's quite a chunky one, so it should keep you occupied for a while. Enjoy!