Fan-TASTIC!

Absolutely amazing, what a gem!
I grew up in the '90s with these sort of games and this is just superb!
The level design is fantastic.
The melee mode is a lot of fun.
There's no handholding, you get clues and directives in certain positions and you get NO magical quest compass or anything marked on map that you haven't already explored! Amazing!
At the time of writing, the dev released the second campaign as a free update. I felt it was already worth the base price of 14 euros, but with the second campaign in it DEFINITELY is worth that price.

EDIT: I have finished the first two campaigns. I don't know if more will follow, but what a ride!
I already really loved the first one, I DOUBLE-LOVED the second campaign!

There hardly is any part to this game that I didn't like.
GREAT:
- There's a demo!
- The gunplay is good, the melee mode is my favourite.
- The level design blew me away and sets a high bar for any other game in its genre.
- The story and lore pieces that you can find are pretty good, which is a big bonus for a shooter like this.
- All the custom mechanics (compared to basic Doom) are really cool. Think inventory, and the way you interact with the world and solve some puzzles.

MEH (not bad, not great):
- The soundtrack has some great tracks, it has some tracks I didn't feel much for, and some of the latter tracks come off a bit bland, repetitive. But all in all it's not a bad soundtrack.
- The GZDoom engine is not built well for very precise jumping puzzles and although this is a really nice part of the level design that makes you feel much more like you're traversing a 3D universe than, say, Doom or Strife, I feel that sometimes it was a tad too frustrating. Especially if you forgot to save just a tad too long ago. It breaks up the flow of gameplay.

POINTS THAT COULD USE IMPROVEMENT:
- I liked the approach of the written notes with hints to what you should do. In hindsight, I like that it has an option of making the important parts clearly marked, or that you can have it as any text and have you really pay attention.
What I think could be better is if all these notes (including things for secrets and optional/alternative stuff) could be saved in the journal. Sometimes i had to revert to looking up a walkthrough because I could not for the life of me remember where that information was, and it's such a huge map in campaign 2!
- Inventory management wasn't horrible. It's very similar to Strife, except sometimes you have a lot of items in there if you tend to hoard them when you don't know when to use them. (hint: use them more often than you would think, you'll run into plenty to pick up.) There are shortcuts to all of the items that you can use in combat, but I personally can't get used to memorizing so many hotkeys.
I wouldn't want to have any less of all the consumables, so doing away with some is not a solution.
Perhaps a 'consumable wheel' would work? Where it slows time and you can pick the consumable from a choice wheel.
- Some puzzles (not always just secrets) and parts of the level design progression are not always very clear. And on the big maps, as much as I love them, it can sometimes feel like you get a little bit too lost. Or the backtracking was simply a tad too annoying. It wasn't bad, but I feel something might be done to better the flow of gameplay when the player doesn't pick the predicted path, since the maps are so open. Example: It took me a looooooong time before I got the pickaxe on Paradise Lost. Which meant I ran across the minable items plenty, but couldn't actually get them. And by the time I did, I have gone through so much of the other content that I completely forgot where it was, and it was a big map to backtrack on.
Another two examples that have minor spoilers I didn't know you could tackle the lust primach in an alternative path until I read it in a guide and it was too late. I was also really wondering if I could do something else than using the potion of life, but I couldn't figure out any other way.
- The companion AI is a bit jank. This is a known issue, the code is written by someone else so I don't know if improvement is possible.

HOPEFUL, BUT NOT NECESSARY:
- Coop. I love coop. Especially with these sort of games. I don't know if it is possible without it breaking things and stuff. (I've read some forum posts about it.) I don't think the game necessarily needs it, but coop-oriented level design would scoop this up even higher for me.
I know the existing campaigns can't be altered to also allow a more interactive coop challenge, but perhaps future campaigns could have a coop mode where the puzzles might need at least two players interact with different things and/or different places?