Just a classic :) .
By today's standards this game, of course, lacks something in graphics and, in my opinion at least, atmosphere.
The balancing may seem a little non-existent but is kept realistic. Of course, a highly advanced alien race does more damage and more kills than a marine with a gun that still shoots metal projectiles.
And in a world where everyone knows, you're there somewhere, and has nightvision or movement radar, obviously isn't that surprised or scared to be charged by an alien. You just have to teach them, that high-tech equipment and almost infinite healthpoint or infite torches to enlighten the entire room (and the room of the player with it), is not enough against cunning, patience and speed.
The Predators do have the most easy game, but are still slow when it comes to changing or reloading weapons, they have to hit when they shoot. Just know what the other races can do and this game may still be balanced.
The singleplayer lacks in "good" tasks, in my opinion. You always have to fight your way through some enemies, which can either kill with one shot (aliens need some tactics!) or not at all (predators are true tanks, aim well and own everything!). The alien campaign is the most fun one, because of this somewhat new feeling of being melee only in a world of ranged weapons.
This game belonged to our top 5 LAN games some years ago and we played many hours scaring our friends with a suddenly dropped alien at the place where your partner was 2 seconds ago. Just brilliant!