The very best single-player FPS made to date.
It's simply superb. It combines every single good aspect of any FPS - vehicles, puzzles, you name it - hones them down with unbelievable polish, and then innovates, but innovates so well that it's hard to think how they could do it any better. Absolutely no part of this game is boring in any manner, you will pretty much never get frustrated, it's simply an incredible ride from start to finish.
First off, the regular FPS part. Naturally, in Half-Life 2 you go around shooting people a lot. HL2 has a wide array of powerful and varied guns, each useful in their own way, that can be used in specific situations to great effect. The shotgun is devastating at close range, and also has a useful secondary fire which shoots out two shells instead of one. The sub-machine gun is a useful, fast-firing default gun, and doesn't feel useless but more of an all-purpose type of gun. The pulse rifle doesn't have much ammo, but has a great disintegrating secondary fire as well as a powerful automatic fire. Each gun can be used in any situation, but they'll best be served if you use them intuitively in situations that make sense. Even the pistol can be a great weapon if used correctly. It's tremendous fun playing each battle, desperately trying to use all your various resources to the very greatest effect against multitudes of enemies, none of which are stupid.
And then the gravity gun. Jason Ocampo and some other reviewers have said that HL2 doesn't innovate, it simply combines aspects of other FPS games that had previously been done before but does them all extremely well. Well, that is true, but HL2 also has the gravity gun, and I defy anyone who claims this isn't one of the best innovations ever in the FPS genre so far. It's not that the physics engine is new or anything - after all it's basically the physics from Max Payne 2 - but it's the way they're used that makes this game special. In other games with physics, you may have fun knocking over boxes by running into them or something, but it's really more of a fun thing than anything to do with gaming. In this game, physics can be at the very core of the gameplay, and you can use the grav gun alone all throughout the game if you like, although it isn't advisable. The grav gun can be used to pull cover away from enemies and shoot it back at them, send sawblades spinning toward zombies, or even to paint headcrabs white with paint cans lying around. The possibilities are simply limitless in battle. The grav gun is also used to a large extent in puzzles, which are fun but challenging interludes from the breathless pace of the action. In my opinion, I concur with Sluggo from Gamespy when he says that the grav gun is worth the price of admission alone. This is simply an unparalleled experience in gaming even nearly a year on, and you won't find it anywhere else.
But HL2 isn't just puzzles and action. There are squad sections when you control some comrades against combine forces and the terrifying striders. There are prolonged vehicle sections when you pilot a boat around or a buggy, desperately racing (and killing) Combine gunships as you go. There's even a much-heralded and awesome section in which you command the antlions with some pheromones. What's really remarkable about HL2 is how it pulls it all together, there is simply no weak part to the game.
The graphics are stunning still, arguably the best graphics made in 2004 (some claim Doom 3, and I'm still undecided), but even more impressive is how they use the technology with the landscapes, whether it's the menacing Citadel that looms in the background or the dark, oppressive Ravenholm which is bathed in shadows and flickering with fire. The art direction in all is extremely impressive, fitting enough as it won the award for best art direction for 2004. As an added bonus, the graphics scale superbly onto lower-end systems.
The sound is terrific, with each gun claiming a unique sound, and the music, sparingly used, kicks in to push you, making the game urgent whenever necessary. While you may regret the lack of a big score a la Halo, HL2's menacing atmosphere simply wouldn't be the same. The focus isn't on huge battles all the time with incredible scores, although you'll certainly get both.
The game is endlessly replayable, and I constantly feel compelled to redo chapters just for the fun of it despite having finished the game already.
Now the multiplayer.
I never played Counter-Strike back in the day, and so when I booted up the redone (and free with HL2) CS: Source I was highly disappointed by the completely unfriendly atmosphere of all the experts and the altogether unwelcoming aspect of the gameplay and community. This is supposed to be one of the best multiplayer games ever created according to many people, but even they will admit that it's hard to get into. For me, that's part of the criteria, but if you played the original CS and loved it, you will love CS: Source as well, as it's basically the same game from what I'm told.
Another free facet of HL2 is HL2: Deathmatch, a terrific multiplayer game with the grav gun and of course regular guns. While Team Deathmatch is certainly a stronger game than Deathmatch for HL2, they're both very fun, and there's really no multiplayer like it. Unfortunately, this is inexplicably and always has been extremely underplayed, especially when compared to the ever popular CS: Source, so it's really a shame that this has gone unnoticed. Also, there are other multiplayer games better than HL2: DM, like Battlefield 2 or UT2004 for example, but not by much, and it's worth playing just for the experience of using the grav gun against other people.
HL2 is pretty much an unending thrill ride from start to finish. The only possible complaints you could level would be about the loading times, which are a little too frequent, and while the guns are all extremely well balanced there's not much new except for the crossbow and pulse rifle, and even those are very similar to previous guns. Plus, other than the grav gun, it never innovates tremendously other than its ability to cohesively meld aspects of several different types of games all seamlessly into one FPS. However, as previously mentioned, the grav gun alone is simply incredible - an experience not to be missed. I totally love HL2, and I have no doubt that any person mildly interested in action games and especially FPS fans should NOT miss out on this if they haven't finished it yet.