Just fantastic, it couldn't get much better than that.
+varied enemies and weapons;
+really, really creative level design;
+very solid, fast and precise gameplay;
+beautiful art design;
+great boss fights.
-not the best story telling out there;
-somewhat generic uninteresting soundtrack;
-linear level design when compared with older quakes;
-OK vehicle sections.
When i started playing quake 4 i felt a little disappointed that it doesnt work like quake 2 or quake 1, you dont go walking around a huge map looking for keys and buttons to press, you need to find keys and buttons on quake 4, but the map design doesn't let you think by yourself, it just points you the right rirection to go, this kinda feels like the game is calling me "stupid" telling me that i cant find myself on a big map. But anyway, even with this huge change on level design, i felt really entertained with the enemies, they have varied attacks and you will find yourself switching between weapons all the time because some weapons are good against some enemies, some weapons are good from short range, some are good from long range, some weapons are good with a group of enemies, and so on.
This is the kind of game that when you put at the highest difficulty, it will not be hard because its unfair, but it will be hard because it will require you all your skills, the gameplay is so precise you can finish the game on the maximum difficulty without taking a hit if you are good enough for that, the game puts you on control.