Half-Life 2 is not the greatest game ever made. It is, however, worth playing if you're a fan of the FPS.
In HL2, you play as Gordon Freeman, a former researcher at the top secret research facility Black Mesa turned freedom fighter when brought into a post-apocalyptic world ruled by aliens known as the Combine. The Combine's goal is simple: gradually assimilate humanity into a mass of slaves that can do nothing but work for the combine. Though the kind of alien ruling over Earth plot has been done before, Half-Life 2's world and story is done correctly and believably unlike contemporaries such as Battlefield Earth (a really bad B-movie/Scientology ad with John Travolta), Doom 2, and Halo 3.
That brings us to the game's next big strength, and that is it's environment. Although the game came out late 2004, the Source engine still looks great in 2008. And the best part; it's optimized very well, unlike Doom 3's engine which can slow to a crawl whenever there is smoke on modern machines. The world around you looks crisp, detailed, and ultimately very believable. The only thing missing that would make it truly immersive is fluent sound to correlate with the well-constructed environments.
Now onto the game's problems. First and foremost, the gameplay. Though the last third of the game is exciting and is packed with superb moments (running with a fully-charged gravity gun at a mass of soldiers is just plain awesome), the rest of the game feels lacking when compared to contemporaries Doom, Quake, Call of Duty 4, and even (dare I say it) the Halo series. The driving sequences that plague a good fourth of the game are repetitive, bland, and simply not fun, especially when fighting an endless horde of easy to beat antlions. And the "puzzles" that come in occasionally during the first third of the game are a joke, consisting of find-the-switch exercises that simply interrupt the pace of the game and do little else.
That's not to say Half-Life 2 is a bad game in any means; it's often just not as engaging as other shooters. However, the last third of the game from Nova Prospekt onward is truly great. Often you fight masses of police and giant walkers with your makeshift weapons during this third, and the action never really stops. And although the rest of the game isn't nearly as engaging, it still offers up a nice world to immerse yourself into, particularly during the first third of the game when the Combine police are trying to capture you, and although you fight a handful of police with a couple of weapons during this part, there isn't a point in the game where you're not feeling like Gordon Freeman, the last free man on Earth.
In the end, Half-Life 2 has enough noticeable problems that more than keeps it from being the greatest game ever made. It does, however, offer plenty of engaging moments, particularly during the end of the game, along with a superbly crafted environment that makes it worth picking up if you're a fan of the FPS.