I just bought this game last week as I saw a video that got me interested in it, and i can safely say, it's one of the greatest games I've ever played. Period.
With its insanely smooth (for its time) playability, the stages, character and monster designs, story, items etc, it all works beautifully together. It's a very fast paced horror meets beat em up, which may sound strange, but it's a very neat combination of gameplay styles and a unique experience that I've not ever seen before. Sure the game has its flaws like the jumping mechanic doesnt always go right and certain enemies are infuriating evasive (looking at you demons and Insects), but it's still truly a fantastic game.
A lot of people complain about a certain mechanic called the "Adrenaline Meter" because it can make the game unnecessarily difficult at times, but for the most part, I'd say it keeps the pace of the throgame flowing nicely. You have to progress hastily to find enemies to fill the meter up so you dont lose health, and I think it adds to the tension that a lot of horror games lack.
From the heroes Ignatius Blackward and Nadia Franciscus, to the "detestable" villain Adam Crowley, everyone is memorable, loveable and adds a lot to the overall ordeal, driven by their motives despite the hellish situations they're forced to face.
In conclusion, Nightmare Creatures is a criminally underrated PS1 game that most people seem to have forgotten over time. If you want a fast paced horror game that's also a hack and slash style beat em up, then this is the game for you. 10/10