I posted my thoughts a couple days back in the recently played thread, so I'll just c+p...
			I jut played Quake 4 for a few minutes, and here's my initial impression...
I 
kinda like it, but I can't help but think my opinion of it is
kinda skewed after playing FEAR. In that game, the combat was
incredibly tense, unpredictable and exciting, and in Quake 4 the combat
amounts to little more than running through corridors and shooting
things that run at you. There's no real reason to crouch behind cover,
no leaning around corners...it all feels very tactless for a game that
puts focus on teams fighting other teams.
It's that very lack of tactical gameplay that makes the whole
team-orientated aspect of the game seem very one-dimensional...the only
reason you need team mates is because you can't kill all the enemies on
screen by yourself; there's no reason to fall back on team mates
besides extra firepower, so you can mow down wave after wave of enemy
grunt that occasionally side steps and runs at you. Sure, thre are
medics, but I didn't feel the need to take advantage of them. It's just
kinda boring. After seeing what FEAR could do with squad AI, it all
felt very deflated and one dimensional in Q4. It doesn't seem as
repetitive as Doom 3's combat system, but it certianly lacks the
tension and excitment D3 created sometimes.
Sure it's loud and pretty, but it doesn't have the same emotional bond
Doom 3 created (which, like it or loathe it, had a great immersive
atmosphere); the combination of silly music, over the top explosions,
mindless enemies and all the ripping off of Aliens (as well as the
stereotypical baddass marine types...must  all commander-types
have that same voice? must the black guy always swear? must the
non-caucasian marines have a RREEEALLYY strong accent?) really
distanced me from what was going on around me; I just didn't care, and
had little reason to continue playing other than to see the next good
looking set piece.
So, get it because it's loud and pretty; that's about all I can say at
this point. Everything else bored me somewhat. Perhaps things pick up
later on, but this is what I think so far (I'm still early in the
game); not the best first impressions.
                                            
                        I Ain't Gettin' On No Plane, Foo'