Good shooter but mediocre graphics...
Throughout the game you spend some time in vehicles, which is a good change to the run n' gunning. Too bad that there isn't a greater variety of ennemies. I also disliked some of the guns at least at the beginning as during the game they get upgraded by technicians. By the way you also get to fight along your squad and the AI of the teammates is - for once - impressively good and helpful.
The difficulty is rather easy in the first half but towards the end you will notice a steep increase especially when fighting bosses on a harder difficulty.
A good thing is that I didn't encounter a single ennoying bug while playing, also the loading times are fast and the automatic saving doesn't interfere with the gameplay by stopping it for a second.
I didn't spend much time on the multiplayer since I prefer UT2004 multiplayer but real Quake 4 fans will certainly spend hours hunting down foes online...
All in all Quake 4 isn't a bad game but not much more than average either.
My major complaint is that the graphics should have been optimised instead they look like a brighter Doom 3 (which is prefered by many fans however). I also disliked some textures and the movements of some ennemies should have been redone to give a more realistic feeling.
On the other hand Quake 4 surprises you with some cool missions and might give you some creepy moments especially when you play at night with all lights off. To run through the campaign will take about 10 hours or even less and if you are not a Multiplayer fan you will lay the game away for some time and maybe replay it later before the mod comes out...