I never bothered purchasing the full version (or pirating it) for years. FPS games are perhaps my least favorite genre (putting me in a very small minority it seems). But I like to play all the classics. With all the love for the 2016 reboot, and fancy source ports keep the legacy of the originals around, I thought it was time to revisit Doom.
I used GZDoom to play the original in 1920 x 1080 with all kinds of modern sexiness (dynamic lights, anti-aliasing, BRX filters for sprites, HD texture packs, etc.) I wanted to preserve the design and gameplay, but spruce up the looks a bit. I eventually compromised a bit on the original aesthetic and went with a completely different HUD though. I know most people in the Doom community seem to suggest that if you're not playing "Vanilla" Doom then you're doing it wrong and they insist all HD improvements make the game look worse, but to each their own. I think this:
http://i.imgur.com/XFbV8JC.jpg
Looks a bit better than Vanilla DOOM:

Does it hold up? Well, it must for some because people are still releasing new WADs, people are still adding features to source ports, people still play the multiplayer, etc.
I was a little disappointed that on Ultra-Violence (second highest difficulty) I could get 100% on every level without too much difficulty if I was patient. Enemy AI is next to non-existent. It is easy to lead enemies to a corner, doorway or bottle-neck where you can pick them off and quickly turn back behind a wall for cover. Enemies have no sense of cover. They're generally not that fast. Very few moments were tense or scary.
A few moments were almost artificially unfair when you start a level with multiple enemies in your face, or you open a door into a room full of monsters directly in your face. Part of the appeal may be the lack of difficulty in a sense. Doomguy feels like a god in the way he can tear through legions of demons and shrug off most attacks. What amazed me is that punches were sometimes more powerful than shotgun blasts. He also runs very fast without ever getting tired.
I conserved most ammo for the big guns and played through all 4 episodes (technically the 4th episode wasn't part of Doom 1 initially, but was added two years later for Ultimate Doom) using only my shotgun and punches for probably 98% of the game. The BFG in episodes 2, 3 and 4 made the end boss fights rather trivial.
The aesthetics were a step up from the odd bright blue walls of Wolfenstein (though I still love the concept of killing Nazis and eating dog food for health). The soundtrack is great. Watching most videos it seems like this is a game meant to be played at a fast pace rather than the safe and plodding way I played it.
My intent was to play all the Classic Doom titles before moving on to Quake. I'm a few levels into Doom 2 currently, but I might take a break before revisiting the series. It was worth playing Doom 1. I'll eventually play the rest, but the gameplay is already wearing thin a bit sadly.
I tried jumping up to the highest difficulty. I started Episode 4 on Nightmare initially. I beat the first level but it was a massive pain starting with a pistol, little ammo and having to deal with a Baron of Hell very early on in very limited space. Later when you unlock a secret it traps you in a tiny room with multiple Barons of Hell. Some people play each level as its own thing with a pistol start. But I like to play games as complete experiences.
Doom offered very little in the way of story or narrative. There was one page of ending for each game with no real prologue or efforts at setting up a story. Episode 4 was a lazy ret-con when Doom 2 starts at the end of Episode 3, but then Episode 4 was squeezed in after the fact with no real explanation.
I know criticism of Doom may not go over well on a site celebrating a love of all things Doom. Let me say I'm grateful for the visual mods and source ports. I do intend to try some fan-made megawads in time. Though I do wish there was a little more to the gameplay of Doom.