Haydee is a pretty good metroidvania, albeit with a few flaws. Navigating the facility is an absolute blast, everything is designed to feel maze-like, but it's rarely ever becomes a hassle to find your bearings. Rooms often have quite clever design, wordlessly teaching the player about gameplay mechanics and presenting just the right amount of information for most puzzles. There are occasional missteps, such as not tutorializing swimming very well (for some reason you have to hold right click to dive underwater), leaving some essential piece of a puzzle hidden in a bizzare spot, or some sections that place far too much emphasis on trial and error. That last part is especially annoying since the game deliberately limits the number of saves you can make, meaning a lot of progress can be lost when you screw up, even if you couldn't predict what was coming.
The gameplay itself is good, but not especially remarkable. You have some basic movement, combat, and exploration. Movement can be slow, with the character needing to complete the full animation for climbing, grabbing ledges, or recovering from a fall before she can move again. Otherwise, the system works quite well, with jumps feeling dangerous but never excessively difficult. The combat is probably the weakest component of the game. The gun models are very noticably lacking in detail, and they sound pathetic. There are a grand total of 2 enemies you can encounter: a fast robot that charges and slashes at you, and a slow robot that walks and clubs you. The walker is almost never a serious threat because you can kill it with 4 easy headshots with even the weakest pistol before it ever gets close. The slasher on the other hand, is an absolute nightmare. It runs as fast as you, and has a much smaller hitbox for headshots - and those count for a lot. Headshots not only save precious ammo, but are the only way to slow slashers down without the magnum. Exploration is the games strongest component. Like I mentioned earlier, rooms are well-designed, but I didn't mention just how many little secrets there are to find. And those secrets are something you'll really want to find, since resources can be scarce - especially medkits. Though the game has relatively few upgrades or special items to find, the ones present are very rewarding to find. Backtracking to the yellow zone to finally brave the toxic fog with my new gas mask was a favourite moment for me. There is one very big oversight though, mentioned in many other reviews of the game: the softlock. In the black zone, if you don't leave yourself a way to return the elevator to floor A, you will be completely unable to access any floors in that zone above you. This is fairly easy to do without realizing, and actually happened to me on my first playthrough. Thankfully, there's a save point earlier, not too far from where you can trigger the softlock. Just make sure you don't overwrite your save though.
Even though I wrote a lot of criticisms here, it's a pretty good game. A solid 8/10 for me. Worth the money.

Good game.