Quake 4 is one of the better examples of a sci-fi first-person shooter done right.
The fourth Quake marks a triple first for the series: it's the first Quake that doesn't bring a new theme to the series, it's also the first Quake that is not powered by new technology (instead it uses a modified version of the Doom 3 engine), and finally, it's the first Quake that is not developed by id Software themselves, development has been carried out by Raven Software.
Story-wise, Quake 4 picks up right where Quake 2 left off. After Quake 2's protagonist managed to kill the Makron, Earth's forces start the final assault on the enemy. You play Matthew Kane, a member of Rhino Squad, and your task is to finish off the alien Strogg once and for all. In order to accomplish this task you will be aided by various soldiers from your side of the barricade, which prove to be quite competent thanks to solid AI. Of course, the enemies are no pushovers either; actually, they are very aggressive and can deal a lot of damage if you're not fast on the trigger and well aware of your surroundings.
But of course, the weapons are your closest friends in this game. And Quake 4 offers a fine selection of weapons. Aside from the enforcer you start off with, every weapon is powerful and useful, no filler stuff here. Raven really did an outstanding job of tweaking and perfecting the feel of every piece of hardware you get to use throughout the campaign. The end result is an arsenal that sounds and feels modern and very deadly.
But another thing the developers got right is the portrayal of this war and the factions involved. Most war games put you against an enemy you know little about... the game just tells you they're evil. Well, in Quake 4 you'll witness just how twisted and sick your enemy really is, so you will not be lacking the motivation to stop them. Some of the things you get to see in this game are a bit rough and disgusting, so I wouldn't recommend Quake 4 to anyone under 18. The humans themselves are quite well portrayed too. There are no cliche army tards who spit out wave after wave of curses to show how macho they are. There are a few jokes and one-liners here and there, but chatter is kept to a minimum and usually about the task at hand. It's quite obvious that Raven understood how a sci-fi war game with horror elements should look and feel like, so hats off to them.
Also, Quake 4 has excellent pacing. You will not be shooting all the time, the action is broken down quite well by great-looking cutscenes, small puzzles or vehicle sequences. Truth be told, the vehicle implementation is kind of minimal; it's not really problematic, but these sequences feel more like an afterthought than a fully-fledged adoption of vehicle-based combat.
Another aspect of the game that needs to be mentioned is the story, which is nothing special (although it has a nice plot twist about half-way through that also impacts gameplay to a certain extent), but your objectives always make sense and are well presented so you'll always know what to do.
Moving on, the multiplayer is quite an interesting case. It's obvious that Raven didn't aim to reinvent the wheel. So Quake 4's multiplayer looks and feels very much like Quake 3, which is anything but bad to be honest. Quake 3 was tweaked to (almost) perfection and was tons of fun, Quake 4 multiplayer is quite similar... the only issue is that levels look nothing like the levels you see in campaign mode. This makes Quake 4 feel like two different games in one pack, but make no mistake, it's well worth going online. Fun is fun, right?
Lastly, the game looks and sounds excellent. My only complaint is that exteriors look a bit weird: the bump mapping on the terrain makes it look real enough, but exteriors seem lit by light bulbs, they don't feel lit by natural sunlight. It's weird, but nothing serious. There are also a few areas where the frame-rate drops a bit, but that doesn't happen often enough to interfere with the experience. The sound effects are excellent (gun sfx are especially well done) and the voice acting is good all around. The aggressive industrial music is also in perfect sync with the overall theme of the game, so that's another plus.
Overall, I have to say, Quake 4 is quite an impressive game. It resembles Doom 3 in some areas, but it's got more personality, and feels well put together. It's never stale, it just keeps going. Go out and buy this game, you'll have tons of fun with it.