If it's old-school action you crave, Quake 4 fits the bill. Just don't expect any innovations or a coherent plot.
Quake 4 picks up right where Quake 2 left off, where a lonely marine defeated the strogg's leader, The Makron. As it turns out though, the Makron didn't die (or maybe he was revived?) and he's still commanding his forces to destroy mankind (actually they want to turn humans into their own soldiers). So it's up to a new marine called Mathew Kane along with other marine squads to put a stop to their plans. From here on you take on with several missions that have the ultimate goal of disabling the strogg's communication services as well as destroying their core system. An interesting twist occurs along the way in which Kane is kidnapped by the Makron and turned into a Strogg (the stroggification process is by far the most memorable moment in the game), but this hardly alters the core gameplay.
It's clear the developers wanted Quake 4 to have a decent story but sadly they just wasted the one chance they had at making Quake's plot not just another generic shooter tale. Everything else about the story in Quake 4 is forgettable, from the cardboard-cut characters, to the redundant cutscenes, to the many plot holes (how is it that some marines instantly recognize Kane in his strogg form even if they didn't know he had been stroggified?), to the ridiculously cheesy ending. If you intend to enjoy Quake 4 you'll have to have a high tolerance for a poor story.
The gameplay itself is also rather generic but it's very serviceable. It's essentially a blending of Quake 2 and 3's best elements, something that works wonders for those who are craving for that retro feel. The campaign is very straightforward which translates into plenty of "boom, headshot" situations as well some entertaining boss fights. The level design is fairly intuitive and manages to offer enough different sights to remain fresh till the end. There are a few sections that require some excessive backtracking, but they're the exception rather than the norm.
While you'll spend the majority of the game taking on the Strogg by yourself, during the early stages in particular you'll get to fight side by side with plenty of marines. This is actually quite neat because unlike in other games, the AI companion is very capable of providing good support. You still have to carry with the bulk of the action, but it's good to see that your squad mates aren't there just for show. Some of them even provide extra help, such as the technician and medics, who can heal you or replenish your armor as you endure damage from the Strogg.
During the campaign there are a few situations when you are challenged with a puzzle sequence, but they are so rudimentary they might as well not have included them since all they really do is interrupt the flow of the game. There are also a few mandatory vehicle sections, which surprisingly turn out to be quite excellent. They're really basic, but they provide some welcome variety to the game (as well the chance to humiliate the strogg).
The multiplayer is quite literally Quake 3 with Doom 3 graphics. The same old maps (albeit slighly modified and some from Quake 2) along with the same old arsenal and that same old commentator ("2 frags left..."). It sure as hell remains fun though, but if the only thing you're interested is in the multiplayer, you're probably better off sticking with Quake 3.
Since it's running on the Doom 3 engine, Quake 4 sports the same spectacular visuals as that game. In fact Quake 4 has a somewhat sharper look along with better lighting effects, which makes the indoor sections very impressive. Outdoor sequences aren't as impressive though, seeing how the distances aren't actually rendered in real time, using instead some very low res static backgrounds. The enemies are all well designed and animate fluidly, some looking quite menacing (the cyber lady from hell in particular).
The audio is for the most part very well done. Weapons don't sound realistic (then again, this is a sci-fi game...) but they still translate a sense of explosiveness. The same goes for all the ambient sounds, from explosions, to the cries from your squadmates as they're getting killed, etc. The one aspect where the audio falls short is in the voice acting. Perhaps it's the cheesy dialogue or maybe the actors were just bored (or embarrassed), but there were times when hearing out conversations was almost physically painful. The same goes for the ridiculous strogg voice that keeps on issuing warnings to the strogg army in english... aren't the strogg supposed to have their own language?
Regardless of all the criticism I've dished out here, Quake 4 was still plenty of fun. It's a shame that all the inclusions made into the game to try to make it a bit more memorable end up working against it (particularly the ridiculous plot). Perhaps had the game gone completely retro it would've been more fun. Still, if it's crazy balls-to-the-wall action you're looking, Quake 4 is your game.