Don't buy this for the campaign, and if it is online play you want then just shell out the extra 20 bucks for COD4.
This game to was almost painful to play. I started the first mission on the easiest difficulty and tried several times to beat it but was unable to, and I assure you that it's not because I am bad at shooting. I have beaten the entire COD4 campaign on Veteran and I am level 45 in Team Slayer in Halo 3. The problem for me was 1) the reticle of your gun is WAY too unstable. I don't know what the US Army was thinking but the gunsight moves up and down at a ridiculous pace, faster than any I have seen in any shooting game before by far, which makes it too difficult to kill enemies. 2) The AI is absolutely HORRIBLE. The enemies walk right to you and your army soldiers, and there is no feeling of reality while playing. On the easiest difficulty of the first mission I stealthily tried to come upon an enemy and immediately he knew where I was even though I had done nothing to arouse suspicion and he had been turned the other way. One shot from him and I was down... which brings me to the friendly AI. Your fellow soldiers walk at a snail's pace and you have to stay with them. They do nothing to help you kill enemy soldiers and only hinder you, and when you are shot and downed your friends take way too long to resurrect you, if they even do. 3) No checkpoints. 4) Although this isn't really a problem, it is fairly annoying to me. When you kill a person they just sit down on the ground. I'm assuming this is all because the Army doesn't want to support showing blood or American soldiers killing people and if that is the case I respect that they are trying to appease the pacifists, but this is a FPS game in the United States, and to me there just isn't nearly enough action. It gets very tedious seeing enemies simply sit down when they are "killed." The multi-player facet of this game could be much better, and does not in any way compare to that of other leading shooters, i.e. Halo 3 and Call of Duty 4.
Unless you are Bill Gates, do not buy this game for the campaign, for you WILL be disappointed. If you want it for multi-player, I would HIGHLY suggest finding the extra cash for Call of Duty 4 or Halo 3, and if it is realistic shooting you are looking for then get the extra 20 bucks to buy Call of Duty 4 because its multi-player is amazing for Army-type online shooting.