Played 1st level of the demo on 2nd highest difficulty. It was looong, about an hour to complete, although a lot of that is because I got lost and/or missed switches and other objectives which open up different sections of the map. Level flow is a bit lacking in that regard but overall the map is varied and fun to explore. Lots of monster closet hordes triggered whenever you flip a switch or pick up a key. Only three weapons (well OK 4 with melee) in the map, but they are all pretty satisfying to use. The shotgun manages to feel powerful and have a good fire-reload-fire rhythm to it without also feeling exactly like the Doom shotgun. Ammo placement is sufficient but scarce enough to necessitate weapon switching and conservative shooting, encourages but does not require exploiting monster infighting.
Enemies all have distinct roles and behaviors which are roughly analogous to Doom's roster, though combat often feels more build-Engine-y; I think this is because enemies often move faster and more erratically, or have variable movement speeds, and there are more hitscan or fast projectile enemies, plus the occasional swarms of fast moving, flying enemies which also shoot projectiles at you.
Ammo scarcity notwithstanding, difficulty is overall pretty modest and I was able to comfortably play through the whole map while only allowing myself one savegame each time I acquire a new keycard, which is the method I use for most Doom maps.
Supplice makes use of Marathon-style Terminal Screens where you read info-dumps of plot exposition or mission objective updates delivered by a cast of Sentient AI characters with mutally exclusive worldviews and motivations. Some terminals open doors or activate new areas of the map after you read them, some instruct you to go to specific locations, some are purely for story. I loved the Marathon series and so far this has a similar flavor, but the quality of the writing will make or break this over the course of a full game, and already I'm finding the terminals a bit verbose to the point where I caught myself skimming most of the text. It's a demo, so without preceding and followup levels and accompanying text, everything is a bit decontextualized which makes it harder to give a shit; might be fine in the full game or might be a huge drag.
Initially downloaded the demo because I had it mixed up with Hyperviolent, which at a glance looks similar, but seems more interesting with its combination of Doom-style FPS and 6DoF piloting sections ala Descent:
https://store.steampowered.com/app/1409200/HYPERVIOLENT/
Unfortunately the demo of Hyperviolent doesn't play nearly as well as Supplice, though I think there's a lot of potential.