Just pure fun violent action. Challenging on higher difficulty settings. None stop action that introduces new, bigger, badder demons as the game progresses. Great fun right from the start. Good pacing with lots of extra community made content, and a pure arcade mode where you play each level (you can choose which one) that awards points for various actions, with the possibility of big multipliers for good players. After the level is finished you can see your score and compare against a huge user base.
But, the main thing for me was the excellent campain.
With a decent PC and GPU the graphics are really very good. Very smooth and looks great on standard 60fps monitors, but butter smooth if you can get it running at over 100fps. Never experience a single crash in the 150 hours or so I have spent playing the campain and other modes.
If there is a negative it's the multiplayer, but never mind, well worth it for everything else which is very professionally implemented.
A classic to be sure. A must buy for all FPS action game fans, and one of the best there is.

Update: May 2024 - Playing Doom Released in 2016 How is it 8 Years later?
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Well I started a new run through of the campaign a few weeks ago.

I can honestly say this game is still just brilliant.

It doesn't have the "wow" factor re: graphics anymore of course, but it still looks darn good, and better it's really well optimized. Anyone with an older rig, Say Ivy bridge CPU with a Pascal GTX 1080, or lower spec new rig, or AMD equivalent should have no issues running it on High.

Except for that, 8 years on and this game is still just as hugely enjoyable as it was at launch. I only play the campaign btw.

Strongly recommended for anyone who hasn't played it yet, or didn't purchase as to play with best settings back in 2016/2017 needed a high end rig to get the most out of it.

Now anyone can play, gaming laptop with dedicated GPU should be fine too.

Bottom line. IMO, it's just as enjoyable now as it ever was.

This game is the real deal, and anyone who likes First Person Shooters should get it, if missed it for whatever reason.