Honestly, the best Alpha/Early Access game you can buy.
It has the most content.
It is (one of) the most playable.
It has one of the most dedicated developers.
It has a pretty sizeable and dedicated playerbase.
In a nutshell, it's minecraft meets DayZ. That's probably the way you're sick of hearing it described as, but it's appropriate.
It's a first person zombie survival game that you can play with/against other players online, with a fully fleshed out crafting system that allows you to make weapons and craft incredible buildings. Though this game has a more "realistic" way of building houses than Minecraft does. Houses actually look like houses, and they even have structual integrity, so if you build a house without much support, it will collapse.
Walk on an old, decaying bridge? It will probably collapse.
Try to build a single line of wood as a temporary bridge? It will fall down after about 3 or 4 blocks.
Mining looks great. It looks like you are actually creating a cave, with smooth walls and dizzying geometry. However mining has had some issues over the course of development. It's been "nerfed", made too abundant, nerfed again, and sometimes even being made impossible. Be warned that mining in this game is a bit glitchy, however this shouldn't come as a surprise in an Early Access title.
The game used to have a blocky asthetic, but the game has since had an update that makes the terrain smooth with rolling hills. It looks great, and it's not even done yet as the developers are working on random generation and on smoothing out the terrain even more.
(SEE UPDATE AT BOTTOM ABOUT RANDOM GEN) Oh, I also need to mention, at the moment, there is only one map. It's a pretty decent size, but you'll see most of it within a couple playthroughs. Building your own structures, especially when playing in an online server, does change it up however.
This game can be played in single player as well, or with a private server. That's another thing great about this game: Options.
There's literally an option for just about anything.
How durable blocks are and how long it takes to mine them.
How many zombies spawn.
How much damage the zombies do to you.
How much damage you do to the zombies.
How much loot spawns.
How many players can join your server.
PVP on or off.
And many other options.
This game can be just about any way you want it.
Some people will point out that options have been both added, and removed, and sometimes options don't have much of an effect at all. Again, be warned, things are constantly being changed, including the amount of options and their effectiveness. There's a chance an option you really like having may be taken away, or accidentally broken. It's all a part of the process of making the game balanced and challenging, while also allowing you to create a personal experience.
There are a few mods, and a few custom builds, but those are pretty few at least until the game becomes more complete. I'm sure once the development moves futher along, you'll see a lot more custom levels and mods being made and updated.
Combat is bit floaty though. As it stands now, melee combat doesn't have that much impact, and guns need better sound. However, usually combat works. Guns shoot bullets into things, melee weapons bash things, but this can be temporarily broken during any update.
Crafting is a major component of the game. You find resources, either through loot in the environment and buildings, or by getting your own from mining from rocks and trees. Crafting is done in a grid fashion, which will look a bit familiar, in that you can place certain components in a certain way to create something. However, it's not all guess work. It's usually outlined what you need and where, you just need to place it. It's a bit simplistic and easy this way, but it works.
For now, 7 Days to Die is a fantastic experience. It still has bugs and optimization issues, such as mechanical bugs, sound issues, and sometimes even having parts of the game you really like being taken out or being made useless. It's part of the process, but overall it's the most complete of the Early Access titles that you can find on Steam, and continues to be updated pretty regularly.
Update: Random Generation. This game NEEDED this. Like most things, it's not perfect, but it's a welcome edition. A random seed is generated depending on the name you give the world, and the results are impressive. You get rolling hills, random buildings and points of interest, large cliff sides, both deep and shallow areas of water (which also added the ability to finally "swim" underwater with neat effect), and a large central city to explore.
The ability to randomly generate worlds now alone has potentially increased the replayablility of 7DaystoDie by countless hours. My first couple hours with random generation I just spent exploring around to find all the nice looking terrain.