Okay, bottom line is, buy this game for the single-player, definitely. Snap Map has potential, and multiplayer is simple, and decent.
A caveat. I am biased, and there are aspects to this game where I am ambivalent about, so it's best to focus on what I write, not the mark I gave it. I somewhat want to give it around an 8.75 if I could, but there are so many idiotic reviews that I need to "fight back" (do some people even know what an 3/10 game is?), and id Software really deserves all the praise they can get because they are clearly talented and it would be a shame if their sales efforts were thwarted because of the ramblings of a few hundred vocal... morons, for lack of a better term.
The game is essentially broken down into three parts: single-player, multi-player, and Snap Map. In fact, you could almost treat this as three separate products.
Single-player
This is clearly the star of the show. id Software has a reputation for making addictive single-player campaigns, and given their reputation as the originators of the first-person shooter genre, they have their work cut out for them.
They have not lost their touch. Yes, in the past few years, they have been losing steam, but they were not out of the fight. In this campaign, you can see that id still has what it takes to make a solid single-player game. When I played the game for the first time, I was immediately drawn in. The atmosphere was gripping and the game just throws you into the thick of combat without giving you a hand to hold.
I could not stop playing. In fact, on the first day, i stayed up until 4 AM playing single-player. Now, I am a responsible guy, but I felt my discipline being test, and I could not stay away from this game. You know a game is good when you can't wait to get back from work and just lock yourself in your room and play it. You know a game is good when you neglect your social responsibilities.
The game's "push forward" combat system is just ingenious. Basically, you have a health meter, and killing enemies drops health packs for you to pick up, subconciously encouraging the player to stay aggressive to stay alive. It's a deceptively simple, yet effective way for the game to stand out in today's health-regen saturated market. Instead of having to wait behind cover to get health back, the game stays true to its roots and relies on a health meter (I don't think id has ever used health-regen. That one Wolfenstein game doesn't count because it was Raven Software). I find this ingenious because it's innovative, but not in an "obvious" way. I like subtle innovations.
Additionally, the exploration is fun, and given the gorgeous environments, a visual treat. The upgrade system is a welcome addition, too. The guns feel great, and the enemy AI is pretty aggressive. (not necessarily ground-breaking, but what do you expect from instinct-driven demons?). I find it to be relatively impressive, actually. But this is hard to explain - don't focus too much on this point, either.
The sound is immaculate. Everything from the guns to the soundtrack shows that id Software crosses their T's and dots their i's. Since I'm a metal head, I paid particular attention to the soundtrack. It's the pump-you-up kind of heavy metal music, and the fact that the music is tied to the combat in that it is a signal to how many enemies you have left is really a nice touch. Small touches like this show that the game has high production values.
The story, despite being not emphasized, is actually quite gripping in my opinion. Now, because it's not the point it should not be a point of focus. In fact, id purposely neglects storytelling in their games sometimes because they just want to focus on the action (Quake, Ultimate Doom, etc...). A good game does not necessarily require a story, and it's probably best if you ignore it. In fact, I felt that the pacing was kind of "off". But again, don't focus on this point too much. And definitely don't give it bad impression because it's not as strong as Half-Life or whatever, although I found myself attached to the characters to a degree, especially the Doom Marine (which is impressive given that he doesn't talk at all - it's his mannerisms).
In a nutshell, the single-player is awesome. In fact, after beating the game, the first thing I did was start a new playthrough on the next harder difficulty because I was dying for more. There's just so much content, and so much to re-explore and enjoy. When I finished the game, I was generally upset with two things: the weird, unsatisfying pacing of the story, and the fact that it ended. Yes, I genuinely had that feeling where you are paradoxically upset and satisfied at the same time because game ended and you want more.
Multi-player
This is where thing's get controversial. I find it pretty fun, but at the same time, I agree with a lot of reviewers who say that it is clearly the weakest aspect of the game. I find it simple in a good way, but at the same time, it feels too simple, almost as if it lacks "substance". It's hard to say, I can't put my finger on it. At the same time, it's not as bad as a 6/10 or even 7/10 as some people put it, but it also doesn't have that addictiveness that Battlefield or even Call of Duty multi-player has.
However, it has enough content to add replay value to the game, but the consensus is, it is clearly not the focus of the product, and I am apt to agree with those reviews. Again, the star of the show is the single-player.
Snap Map
It seems to early to tell, but my hunch is that it has a lot of potential. I expect this to keep me incredibly amused and entertained for hours to come.
So, all in all, this game is worth your money. Do not listen to those people who give it a 2/10, or 4/10. I understand if they give it a 7/10, but a 2/10? If you read most of those reviews, the reviewers peg the game for the dumbest reasons ("There's no health-regen" Why should there be any? "It feels repetitive" How?)
*PS, my first playthrough lasted 16 hours, but I was playing on above normal. For the average gamer, it would probably take around 13 hours.