Half-Life 2 is simply one of the best-crafted experiences there is.
Set an indeterminate amount of time after the first game, Half-Life 2 finds you called upon by the mysterious G-Man once more, as you awaken on a train headed to City-17, one of many cities on Earth, now ruled by aliens known as the Combine. It isn't long before you get your suit and crowbar, and are off to save the world once more. It's definitely more complex than that, but I wouldn't want to spoil anything.
What really sets Half-Life 2 apart from almost every other game on the market is its lack of cutscenes. The entire game, from the moment you start anew to the very end, is shown from Gordon's perspective. All the characters will talk to you and each other, and relate what's going on. This helps invovle you in the game, and really create a sense of immersion in the events. It's a true shame that this is really one of the only games since the original Half-Life to do this.
It's quite an adventure. You move from the city to the surrounding countryside and back again, going through a series of diverse and exciting encounters. Valve does a masterful job of mixing up both the settings and the gameplay, with sections involving vehicles, squad-based fighting, and solo combat. Although parts of the game drag a bit, you'll hardly get bored.
What really sets Half-Life 2 apart from most games on the market is its sense of mood. You really do get the sense that you are in this city in quasi-post-apocalyptic earth. Houses and buildings are scattered around, often requistioned by the Combine for their own ends. There's a real feeling of melancholy in your adventures.
Most of the weapons you use against your foes are pretty standard. What stands out the most is the gravity gun. This allows you to manipulate just about any object in the environment and grab it or throw it. This is not only useful in a number of ways in combat, but it's also used for puzzles. It helps make Half-Life 2's combat stand out from many games like it.
The graphics, both technically and artistically, are top-notch. The framerate stays solid, and the lighting is great. The game does a good job of displaying realistic indoor and outdoor environments in many different settings. The characters, especially those that you talk to, are wonderfully animated. The faces give you a great sense of the emotions that the characters are feeling when they talk. The only real problem is that loading times between areas in the game can be rather long, and hurt the otherwise excellent pacing.
The sound is very good. During combat-intensive sections, fast-paced music kicks in. Otherwise, the soundtrack is realtively non-existent, which is fine. The sound effects are very good, however. You'll know your enemies well when you hear them, and they can create their own sense of dread without need of much help from the soundtrack.
It's impossible not to recommend this game. The single-player experience is simply unsurpassed just about anywhere. It only leaves you wanting more, and the amazing mod community delivers that. There are numerous single and multiplayer mods available, and Counter-Strike: Source comes with the game. Get it. Now.