This would be the one game I played in its entirety more times than I can remember. From the first time I played it on the PlayStation back in 1999, I come back to it every now and then when I just want to let myself loose. And it feels great every time, no matter the platform - Nintendo 64, PlayStation or PC. Although, many people were disappointed it's not a true sequel to the original Quake - it is the first entry in the Strogg saga.
Some time in the future, the Earth has been invaded by the Strogg, a race of cybernetic aliens, whose past and origins aren't quite clear. They don't take prisoners, they take unwilling recruits. Dead or alive, they will serve as an extension of the ever growing Strogg army. The Space Marine Corps have decided to show the Strogg they won't just sit around while our planet's population is being decimated by the alien forces, and have launched a massive counter-assault on Stroggos, the invaders' home planet (or, at least, their current home planet).
As the operation goes downhill the minute it started, the player assumes control of Bitterman, one of the few marines left alive on the surface of Stroggos. Armed with whatever he finds along the way (and he will find a plethora of weapons), Bitterman must cut his way through Strogg defenses all by himself, and try and bring the Strogg forces down.
The unique atmosphere of the game will keep you on your toes all along, as the Strogg fall under your sights one by one and you complete objectives sent by the Earth command.
The only downside of the Steam version is the lack of the original soundtrack, a very strong heavy metal score composed by Sonic Mayhem, which really gave the game a unique feel. This is something that should really be rectified in an update or a patch, as playing Quake II without the Quake II music is like riding a horse, naked. It just doesn't work. Since the original game read music directly from the CD, I don't think it's such a big problem to have the OST in a compressed format, along with the game. Or at least have the CD image of the game available for those who purchase it, so they can burn it and enjoy the full Quake II experience.