About bloody time this turned up on Steam.
I played this a ridiculous amount when it came out. I never did conquer the singleplayer mode - since it's apocalyptically tough - but I had so much fun with the Stranded map in the skirmish mode that it's almost criminal. I used to just stick on the God mode, chuck myself down the shaft leading to the arena at the bottom, run like hell to the Marine outpost and then just slaughter aliens for hours on end. I remember trying that level 'honestly' a few times - very rarely did I make it to the outpost with all the good weapons in it. The first time I did make it, I was so relieved that I had gotten there intact that it took me a few seconds to realise that while the output may have sealable doors, it didn't have windows. Of course, by the time I realised this, there was an alien on the ceiling above me, about to take a great big CHOMP out of my head.
Skirmish mode was always best to play as a Marine, though. It's freaking impossible as a Predator, as you just run out of ammunition for the speargun and energy for your other weapons so quickly. Being a Predator in the singleplayer campaign, though. Damn, that's fun. You've got all the vision modes and sound effects from the film and you really feel like a huge baddass alien. Well, you would if you didn't spend most of your time cowering in a dark corner waiting for your energy to recharge all the time... That said though, playing as a Predator did blow my socks off, mainly because it was one of the first game I played where you could zoom in your view on enemies. I used to take pride in one shot kills with the speargun that would pin the heads of marines onto the wall, leaving them glowing brightly against the dull metal in the thermal vision mode. Again, it does get ridiculously hard when you start fighting aliens, basically because it throws too many enemies at you simultaneously and you run out of energy too quickly, even if you're really careful with the way you use the plasma pistol - energy management as a Predator is the biggest challenge in the game, I think.
Speaking of challenges... playing as an alien is probably one of the most disorienting experiences I've had in a videogame. It's like trying to correct a flat spin in an aeroplane with your eyes closed. The aliens are just so damn FAST, and the way you run across walls and ceilings like they were the floor is just insane. Speed is life for an alien - you can't play like in a normal FPS, because you're just so fragile and the only way for you to replenish your health is for you to make a HEAD CHOMP (Best done on scientists, for obvious reasons - though props to you if you can manage it on an alert marine or predator).
The thing I love best about Aliens vs. Predator is that it's one game, but it gives you three very different experiences. In the same way that some people argue Thief is a roleplaying game (that is, you have to act as Garrett would act to succeed in the game), AvP is similar: you have to play each role (Marine, Predator or Alien) as if you were genuinely in their head. Marines need to be ultra-paranoid because the entire world is out to make them their lunch and their only defense is their high tech array of weaponry. Predators also need to play to their strengths - ambush from stealth and try not to get outnumbered or surrounded - if they want to be assured of victory. And Aliens need to just run like hell, stay on the ceilings and walls and CHOMP-CHOMP-CHOMP their way to supremacy. Playing as an alien is probably the most rewarding, when you get used to the fish-eye-lens-o-vision and the ludicrous speed, but whichever role you choose, one thing is certain. This game will scare the pants off of you.