I've been playing Duke3D sporadically throughout the years, but the more I played it, the more my opinion of it decreased.
It definitely leaves a very strong first impression primarily due to its level design and environmental design. Levels have rather large areas with lots of verticality, and they're filled with details, making them look and feel like real places. As I mentioned in my Star Wars: Dark Forces review, it's not the first FPS to do it, but it certainly outdid Dark Forces in this regard. The level of interaction with objects in the environment was completely unprecedented for the time, and, like Dark Forces, it plays more like an action-adventure than a Doom clone.
Which is a gift and a curse in this case. Because, while yes, it does get you more involved with the environment, there's still a lot of shooting that you have to do, and here the game absolutely loses to Doom and even Dark Forces in almost every regard. Most of the enemies are either hitscanners, or tiny annoying enemies that are hard to hit, or annoying flying enemies that are hard to hit. If you hated Lost Souls in Doom, you're in for a ride with this one. Doom was smart enough to limit the amount of tiny/flying enemies, and keep hitscanning mostly to the weakest enemies. The whole skill-based gameplay of Doom comes from dodging enemy attacks while shooting them. That's practically gone here, and a medium-strength hitscanner can kill you in seconds. You don't really have any means of dealing with them other than dipping in and out of cover, except of course the mechanics we'd see in cover-based shooters wouldn't arise for another 6 years or so. Similarly, there's no efficient weapon to deal with heavy flying enemies. The light ones you can take out with a machine-gun, but every heavy weapon you have is projectile-based, which means hitting the most mobile targets is an exercise in frustration. Guns like the shrinker and the freezethrower are just gimmicks that fuck with the flow of the game. I mean, why would you want to use a gun that makes you stop and stomp/kick an enemy you already shot?
But by the middle of the game even the level-design begins to suffer. The game in general has some rather cryptic levels, but at that point you start seeing some that are absolute fucking bullshit. Like the kind where you have to press an object to find a hidden door to finish the level. And then I spent like 20 minutes running around the Overlord level, tearing my hair out to figure out wtf I was supposed to do, until I looked up a walkthrough and realized that the game just fucking bugged and the boss didn't appear where he was supposed to.
And of course, what kind of a Duke Nukem review would this be if I didn't address the stripteasing elephant in the room? That is the controversy that has arisen in the recent years regarding the game's treatment of women. There's so much debate out there, with one side calling Duke a misogynist, and the other side treating him like a saint. The phrase "product of its time" has been thrown at the game an infinite amount of times, which I think would actually be appropriate if it was commenting on the gameplay, not its socio-cultural positioning. But, as it often happens, the truth is somewhere in the middle. Is Duke an example of chivalry? Well, no. Is the game's depiction of women problematic? Yes, it is. But is it downright misogynistic? I don't think so. I think Duke3D is largely tongue-n-cheek and feels more like a parody of 80s action movie tropes. That being said, regardless of intentions, the sexualization of women that are victims of horrific alien breeding/spawning practices feels very weird and uncomfortable. And the fact that you can't save them and are doomed to witness their gruesome deaths again and again just feels really out of place in a game with such a light and comedic (barring a few exceptions) tone. And I'm not saying this from the "if I was a woman" kind of abstract perspective. It's uncomfortable to me as a man. But I would say overall the game doesn't really cross the line into objectively offensive.