SiN is a hot mess. Stylistically it apes the self-aware stupidity of the mid-90s action movie (think "Escape from LA" instead of "Escape From New York") and draws gameplay inspiration from a mix of Duke Nukem and Goldeneye 007, combining ego shooting and objectives to complete.
The action is surprisingly fast, kinetic, and fun. The hero, John Blade, moves at a generally satisfying speed, the most common enemy types do not suffer from the bullet sponge bloat of many other games from the era, and the weapons feel effective. The game has a unique armor system in which the head, torso, and legs have separate values and protect those distinct parts of the body. As far as enemies are concerned this simply means you should aim for the head, but as the player it means you have a very robust protection system. The result is that when you're capped out getting shot can result in no HP loss whatsoever until your armor is chipped away. Furthermore, armor pieces can be looted from just about any enemy corpse. Corpses sometimes have medkits on them as well, and the game has an Overcharge system similar to Wolfenstein: The New Order wherein HP in excess of 100 will count in your favor, but slowly deteriorates over time. The mechanic works better here than it does in Wolfenstein's setpiece-driven design. When it works, you're given a steady stream of resources that keep you running and/or gunning and SiN comes alive, feeling like the "true" Duke Nukem 3D sequel that never was.
Unfortunately, it doesn't always work. Specifically when that stream of resources runs dry and the level design stops playing fair, SiN becomes painful. The quality of the maps veers wildly back and forth from acceptable to godawful and back again. Sometimes they forget to give you health kits. Sometimes there's simply no ammo in the level. Bizarrely, at the very end of the game, there are entire levels that just don't have enemies. Sometimes you climb a ladder up out of a manhole and as soon as you peek your head out a sniper that is positioned behind you instantly takes off 70% of your health. Sometimes you move forward and rocks fall on your head, killing you instantly. There were more than a few long stretches of the game where my health was lower than 10% and I was forced to inch forward, wearing out my quickload key. And I must warn you, even on a modern PC there's very little that's 'quick' about the quickloading.
There are jumping puzzles. There are moments where you're forced to take damage. There are barely functional stealth sections. There are parts within these stealth sections where triggering an alarm (almost impossible to avoid) will cause guards to infinitely spawn in. Which would be one thing if they spawned in from some clown-car monster closet and flooded out. But not in the middle of a hallway. Not when I'm looking directly at them magically boink into existence. So yeah, there are bugs, and they rarely favor the player. The physics are constantly fucking up, and fucking you over. Clipping errors will damage you. Destructible environments such as pillars that crumble when shot are meant to look cool, but when the debris falls on your head it's not much fun. Doors will push you around and relocate you if you get in the way of their opening animations. John Blade may be a big muscle shoot man, but in my experience he spends a lot of time being crushed to death inside of lockers.
Each mission has objectives, but oddly enough most of these objectives are optional. Instead, the idea is that completing or failing to complete an objective will have a knock-on effect in later missions. I was playing a digital copy, but I presume the back of the box indicated that this means the game will be totally different each time you play! Unfortunately, the changes are so minute and the game never calls out the cause and effect so unlike, say, Deus Ex, you'd be forgiven for not even realizing that this is a feature. On the contrary, it can sometimes just be confusing. At one point in the game, the focus of the plot is to stop the villain from launching nuclear missiles from an aquatic base. In my playthrough, I found a secret exit through an underwater cave and ended the mission before disarming the nukes. The nuclear missiles forgotten about and the aquatic base behind me, instead I was dumped unceremoniously into the middle of a jungle with no explanation. These jungle missions were awful and brutually unfair, offering zero health pickups, very few ammo refills, tons of giant mutants that tear you apart and take a beating, and a great number of 'fuck you' traps. At the end of the set of jungle missions, John Blade was captured, turned into a mutant, and made to do jumping puzzles for a single incomprehensible mission before being turned back into a human with a magical scientific device before being shuttled deeper into another lab. In this lab the walls were apparently made out of meat and I was constantly being teleported into death traps with no warning and for no discernible reason. Enemies in these levels were fat men in hazmat suits that took three direct rockets to the chest to kill and yet the level designer did not see fit to include rocket ammo in the map. To say this stretch of gameplay was bad is correct, but also missing the point. To say it was confusing is an understatement. It was more like I was on a stretcher in Vietnam and Ritual Entertainment was sending me down Jacob's Ladder.
Look. Like I said before, when SiN works it does work. The shooting is fun enough, it has a really killer soundtrack, and Freeport is a colorful place... when you're not being forced to explore sewers, warehouses, and underground labs made of meat. In these moments it's not exactly spectacular, but it's like a shitty chain restaurant pizza. Are you just not gonna eat it because it's not perfect?
It just... doesn't always work, you know? Sometimes the cheese slides off and you see all the rat turds underneath. Sorry, that's gross.