Long ago, this was an awesome game... but I was also a fan-boy of anything Alien-related. Nowadays, I see clearly.
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Maybe time made me forget about these issues, or maybe I was less critical of games back then, but this game is greatly hindered by these problems, in single player or multiplayer.
The A.I. sucks. It's stupid, predictable, it gets kinda boring nowadays. Humans walk around in strange paths (not completely scripted) and aren't too reactive too their environment. Yeah, it's 1999 we're talking about, but Thief was extremely successful. The Alien A.I. fares no better: they all just run straight at you screaming the same 3-4 sound effects, nothing Alien like about it. They're fast, but as a human or predator, you can always out run them in at least single player (which is pretty stupid).
As long as we're discussing running speed, it is ridiculous that you can out run any danger backwards (and a little side step here and there). This kills any suspense to be had (the game stopped being scary within the first hour), and the multiplayer experience sucks as a result as you have three teams dashing around like lightning bolts and everyone requires too many hits to down. Just think of this game as Unreal Tournament with increase speed, some characters use only a pistol, another only a sniper rifle, and the other a jack hammer.
Lastly, coming back to single player: there is no real narrative going on, and unless you've played through the game many times, it is understandable why many people get frustrated when you don't know where to go next. You get a small written message at the very start of the mission, and then you get occasional videos playing on screens through the map (with terrible acting). Maybe it is to add to the immersion, but just trying to survive isn't interesting if the opponent is stupid and easy to dispatch of.
With that said, not all is bad (my inner Alien-fan-boy acting up again?), as the Alien campaign is well worth trying as it is fun to finally see through their eyes (not to mention wall climbing is at its best in this game). The issues with the story don't really effect them, because how could you really express "SAVE HIVE SAVE HIVE SAVE HIVE" in a more suiting manner?
The Human campaign suffers badly from "where do I go" syndrome. The predator campaign... though I finished it countless times (to unlock the fun cheat modes), I don't see the appeal of playing a space redneck (lets go there an' ge' ta' huntin'!).
Over all, the game is okay.