I originally played this game shortly after it came out, and I have fond memories of the singleplayer, and of playing LAN multiplayer with family. Playing it properly again was a bit surreal, it's the first time I've done so in almost 20 years. SiN was a pretty unique game, quite advanced for an FPS of the era. It has locational hit-detection (on both enemies and yourself), moments of downtime with stealth and puzzle solving, optional objectives with rewards for completing them (such as changes to subsequent levels, or new levels entirely), and plenty of destructible objects. It even has driveable vehicles! (Thankfully only for brief moments, because they control about as horribly as you'd expect.) The game also features fully interactive security consoles, which you mainly use for puzzle solving, deactivating turrets and opening doors. For an (almost)-linear FPS of the late 90s, the game had a surprising number of features and was rather ambitious.
Released less than a month before Half-Life came along and revolutionised the genre, the game doesn't really have a lot in common with Half-Life, because SiN is one of the last of it's kind. It's still firmly stuck in the era of collecting coloured keycards for doors, bulletsponge enemies, and AI that basically has two modes: running around in circles, or standing perfectly still while they shoot you with pinpoint accuracy. The game is so gloriously late-nineties - John Blade, a huge gun-toting mofo with dreadlocks and Lennon glasses, and his "hilarious" sidekick JC the hacker, fight an evil corporation run by a scantily-clad villainess, whose hobbies include world domination and turning people into mutants. It's cheesy as hell, and it makes me wish more games would go back to being as over-the-top and light-hearted as SiN was. But playing it in 2017, my positive thoughts are overshadowed by my criticisms.
The first thing you may notice playing this game today is how unresponsive it is. Movement feels very loose and floaty, and it just doesn't have the tight controls that many other FPS games of the same era do. Despite this, the game forces you to engage in awkward platforming puzzles, mostly in the first quarter of the game, and it's really infuriating. Once you've got past that, the level design becomes less lateral (with a few exceptions) and more closed-off, so traversing the environment is less of a problem, but movement itself never "feels right" at all. It's hard to describe, but I'd consider it my main issue with the game. I found it a constant annoyance in combat, and even just when moving around in general, as I'd occasionally get stuck on the level geometry or find it overly difficult to make a simple jump. There's just nothing refined about the controls in this game, it all feels very archaic, and broken in its own unique way.
To expand on the changes to the level design I mentioned; as you get further into the game the levels become a lot less complex and more... well, dull (which might be a symptom of the game being rushed out to compete with Half-Life), and I found the changes very noticeable. As the environments became less interesting, I started to wish the game would hurry up, and there were sections where I got bored enough that I started noclipping just to get through the level quicker. Lastly, the general difficulty of the game, if we were to normalise it, would be about average; but when SiN does decide to get hard, it gets annoyingly hard. The game is plagued by short but ridiculous difficulty spikes. Cheap ones, caused by bulletsponge enemies, indestructible turrets, and snipers that can kill you with one or two shots, but are placed in positions that make them very hard to see and even harder to hit.
SiN did some interesting things, but I have to say it's aged quite poorly. Even with all my nostalgia, I struggled getting through it at times. I have a soft spot for it, and it deserved much more attention back when it came out, but it may not deserve your attention today. It depends on how much old-fashioned jank you're willing to put up with. I'm usually willing to put up with a lot, but SiN was pushing its luck.