Let's get my opinion across to you as a reader as soon and as clear as possible.
I genuinely love this game.
For an early access game, it feels very polished and content rich. But then, it has been in early access for some time now.
If you like Diablo, you will definitely like this game. Because it offers the same hack&slash loot fest gameplay as that game, but without the frustrating bull****, such as the permanent character damage of Diablo 1 and the excessively restrictive builds (And rare respec) of Diablo 2.
The game offers at present 5 different classes, of which you can combine any 2 of them to create hybrid classes. There is the Soldier (Typical warrior), the Nightblade (Rogue with a melee focus), the Demolitionist (Archer type, but with versatile crowd control options and traps), the Occultist (Debilitation and pets) and the Arcanist (Spellcaster). There are skills in each class that well complement those of another, but a pure class build is just as viable.
There are no skills that are purely support, since the developers have made a conscious decision to make singleplayer just as playable as multiplayer. Instead, most classes offer some form of skill that applies a bonus or ability to you, as well as an equal one to other players and pets, allowing increased power and versatility in multiplayer without compromising the viability of the skill in singleplayer.
It's a lot less restrictive than most similar games in how each class can be built and still be viable, but it is worth putting some advance thought into your build. Skill points can be refunded if spent on skills, but not if spent on class mastery (Which increase your stats linearly and unlock further skills).
Content wise, the game currently contains 2 full chapters of reasonable length and 2 out of 3 parts of the 3rd chapter. A total of 4 chapters are known to be planned for the full release. There is a lot of various equipment to find with interesting items offering unique skills to complement the arsenal of your chosen class. There is also a lot of varied enemies to use your weapons and abilities on, with especially dangerous (mostly) randomly spawned unique Heroes to contest with, each with their own special skills and abilities.
Four difficulties are planned, each which will be unlocked for a character as the previous is completed. At present there is only two difficulties available, Normal and the optional Veteran. These two difficulties can be alternated between at any time from the main menu and share the same campaign progression. Completing the game on either will unlock the next one, once it's added.
I strongly recommend playing on Veteran difficulty in singleplayer, as Normal mode is a tad too easy. In multiplayer, the game scales the difficulty based on the amount of players, making Normal mode offer a decent challenge and making Veteran very challenging.
Enemies do scale to your level, but as I understand the scaling will be lesser in the full release and is only set as high as it is now to offer a continuous challenge for higher level players. Right now, the maximum level is 40, which is still plenty enough to allow the construction of very interesting character builds that do end up feeling extremely powerful in spite of the scaling enemies.
Story wise, the game features an original world by the name of Cairn, with has an Victorian-esque aesthetic and technological level. It offers you the usual swords and shields but also black powder guns, bombs and magic. In this world mankind is at the brink of extinction with the event dubbed the "Grim Dawn" as it's cause, where the otherwordly and malevolent Aetherials and Chtonics invaded the world. The Aetherials seeking the enslavement of mankind as hosts of possession, while the Cthonics would rather mankind be wiped out, as to deny their use by the Aetherials. These two factions are enemies and will actively fight one another, in addition to humanity.
The Grim Dawn led to the total collapse of the empire, leaving only scattered groups of refugees and army remnants trying desperately to safeguard their own survival. You join one of these groups in trying to secure shelter, supplies and to strike back against the invaders and Cairn native monsters threatening you. All in all, it's an interesting universe that manages to put a somewhat new spin on the overused post apocalyptic and dark fantasy settings. The story also feels much more grounded than many similar works, with the player not being a chosen one destined to save the world but is just another survivor struggling to survive. This is reflected in the quests you undertake as well, with quite a few of them simply being about securing the base needs for survival, such as drinking water, food and usable materials for clothing.
Unlike most games of this style, Grim Dawn offers choices with permanent effects on the world. So for example, your choices can decide who lives and who dies as well as influence what equipment you can access. Or they can even turn another faction of humans against you, for siding with their enemies. There is a reputation system, where you earn the trust of factions by completing quests and fighting their sworn enemies, with increased trust letting you access faction unique bounty quests and a store offering equipment unique to that faction. You also accrue a negative reputation with the hostile factions, where you face stronger opposition the more of a threat you become, culminating in them sending especially strong unique enemies to deal with you. The choices and faction system feel very well integrated and can only become more interesting the further along in development the game gets. A sense of personal investment in the world makes it unique from it's genre brethren. Of course, you might make a choice that ends with an outcome that you don't like, so if that's a concern you would need to refer to a guide to make the choice you want.
The game does however suffer from some unfortunate bugs, primarily a tendency to crash. This combined with rather extensive dungeons can make for a frustrating experience where you lose your progress and have to start over. The characters however do save properly though, so it's not an insurmountable chore to go through those dungeons again.
There's a few performance hiccups here and there and some minor graphical bugs as well, but it is an early access title and such things are to be expected. If you can't deal with that, stay away from any and all early access titles.
Do I recommend this game? Yes.
But with the reservation that it is in early access and thus not yet finished and full of bugs. If you can deal with that, buy it. If you can't, then wait.
Important note: If you have a router, to be able to host multiplayer it needs to support Upnp or be able to port forward UDP and TCP ports 27016.