Mutants, gibs, frags. The ultimate fps!
The game has many cool weapons, each with it's unique firepower. None of the weapons are useless (apart from the standard blaster which SUCKS!) The super shotgun, although not the best weapon, is my favourite as it is reliable, powerful, and makes a satisfying BOOM! The enemies are cleverly spaced out to suit the situation. None of the levels have ridiculously large numbers of enemies; harder levels focus on the quality over quantity of the enemies. Sometimes, it is annoying when you are fighting tough enemies with the railgun, then you are forced to waste some shots on the weaker enemies because changing takes too long.
The ai is ok. Most of the enemies just attack relentlessly without considering the fact that they are just about to be "gibbified". However, most of the enemies are mass produced mutants whose only purpose is to kill, so it doesn't really matter if they die.
The maps are cleverly designed. Rather than in the original quake where you follow one path blasting the hell out of anything that moves, you have to get keys and deactivate defenses before you can proceed.
To enjoy this game in its prime, you should run it using the new knightmare engine. This is available for free from many download sites, and fully utilises top spec graphics cards to provide trilinear texture filtering and vastly improved particle effects.
This game has a really cool music soundtrack that gets you in the mood for blasting those mutants! It has the best music in any game I've ever played.
Bottom line: This game will become as loved as doom for sure.