I really, really tried to like this game. Played it up to the haunted cathedral on expert, (a level short of halfway through the game) but I just don't have fun.

First the good stuff: While I used Tfix, the HD mod and a sound enhancing mod that cleans up the original sound files, it aged pretty good in the Graphics and Sound department, especially the overall audio design. With headphones you can tell pretty well how far away or how close an enemy is, with doors being open or closed influencing this. Albeit you and the AI can sometimes hear footsteps from a floor above or below you, which can lead to awkward situations.

The story was pretty entertaining, even if it wasn't anything groundbreaking.

The stealth is.. a mixed bag, honestly. How loud you are is determined by the surface you're walking on and how fast you are, with faster movement speed causing more frequent steps. Normal walking and below is pretty silent on carpets, but if it's a metal surface or a tiled floor even crouch-walking will alert nearby guards.
People get around this by simply tapping or only briefly holding the Forward Key, stopping before Garrett finishes a cycle and would cause a footstep noise. A lot of people seem to think that this is a feature, but to me that seems more like an exploit. Up to you if you make use of that technique.
One technique I don't really consider a exploit is hitting someone in the air with the Blackjack. Landing after a jump makes a lot of noise, but if you can bridge the gap between you and your Enemy with a jump and hit them during it, then they don't have enough time to react and turn around and be alerted, failing the attempt.

The vast majority of players suggest to play on Expert difficulty, even on your first playthrough. This is because higher difficulties add extra Objectives, that are otherwise blocked off in lower difficulties and lead to missing out on Content. I largely agree with that sentiment, given that the AI for guards doesn't change between difficulties, and the only changes are higher Money amount required + extra objectives and having a bit less Max Health. Take note though that the game will not allow you to kill human enemies, meaning you have to either avoid them or knock them out via the Blackjack, Flash bomb + Blackjack Combo or the Gas Arrow. Other tools like Fire Arrows or Mines are better used against creatures or the undead.

Level Design is another mixed bag. I preferred the more grounded levels with less supernatural elements over the other kind. Just wasn't really a huge fan of The Bonehoard, or the weird The Sword level. But also ♥♥♥♥ the Thieve's Guild level, it was one of 3 Levels added with the Gold edition and is largely hated by people for a good reason. Feel free to use CTRL+ALT+SHIFT+END to skip that Level. The maps can end up feeling like a maze, and your map is very basic and only shows you the general location, along with giving you a compass to find your way. For the most part it wasn't too difficult to find my way around, but at times I felt stuck because I simply missed that one critical path while running in circles.

The AI in the game feels pretty dumb, they react to seeing glimpses of you and noise, but they don't really give a ♥♥♥♥ about you extinguishing torches, leaving open doors, stealing items, slashing banners unless in a few instances where it's scripted. Sometimes it feels like they arbitrarily turn around and spot you for no reason, feels a little bit like it's a lot of trial and error.

The platforming and movement related to it feels really bad in this game. It's really finnicky what makes Garret want to climb an object instead of bouncing off in a weird manner. Sometimes you get stuck on an ankle-high step, which especially during combat can feel awful. My recommendation is to make good use of your Quicksave and Load to mitigate this.

I can appreciate that this game in particular did a lot to help create the Immersive Sim Genre, along with creating a good basis for the Stealth Genre. But I don't think I can say that this game aged well.
Maybe my view is just biased after playing through Splinter Cell 1-3 recently again (with 1-2 for the first time.)

Your mileage may vary though.

This is a genre defining stealth fps and here's what you will find:
-An original world with its own lore
-A chance to roleplay as a thief character that feels like a 'real' thief
-Discouraged from killing, encouraged to stealing
-Stupidly complex, strange and fulfilling levels
-Hand drawn and perfectly imperfect cinematic art
-Eaves dropping in every level to collect critical information

REMEMBER TO MOD the game which will fix all kinds of issues like cinematics not playing and the game crashing. There's a big patch you can use.

Best game ever.