DOOM is a classic for a reason, and it's still a blast to play. The combat is fast-paced, visceral, and immensely satisfying. The levels are well-designed, and they are complex without being maze-like. There are a variety of enemies with different attacks and movement styles, and they range in toughness from annoying cannon-fodder zombies all the way to the giant, rocket-shooting, insanely deadly Cyberdemon. On top of all that, the sound effects, gore, and hellish imagery serve to create a sense of atmosphere that nicely rounds out the gameplay experience.
The controls, and the inability to look up or down, give DOOM a slight learning curve for first-time players. However, with just a little practice you'll be fine. Additionally, mods like Brutal Doom bring a lot of cool new things to the DOOM experience, including modern FPS controls.
Simply put, DOOM is a must-have game for anyone who's a fan of FPS games.