Having owned the game for a few days now, and played through the story twice and the multiplayer almost constantly, I feel it's time to give a review!
It was an interesting move for the Call of Duty series to venture away from their WWII 'safety-zone' but then again, we've seen the ending of WWII more times than we've had hot dinners, so the formula was getting a little old. Needless to say, the 'modern' update is refreshing and new.
The story has you playing as both an S.A.S soldier (Soap MacTavish) and a few U.S.M.C soldiers (depending on mission location). Both styles are fundamentally different, with your S.A.S missions focussing on stealth, and point-entry whilst the Marines missions focus on supporting, reclaiming ground, and neutralising confilct zones. Absolutely needless to say, but both sides offer a rich gaming experience and there's fantastic character building on the S.A.S missions; you really feel like part of the team.
I won't go into too much detail on the story, because I don't want to give anything away, it's hands down the best FPS story I've ever played (and yes, it beats Bioshock in my opinion) and it'll give you many many hours enjoyment as you'll want to play it again and again! One technical thing I thought I'd throw in is that the game is recorded at 60fps, so it's super smooth even on 1080i! This is also true for multiplayer!
Moving on, I suppose Call of Duty's crowning glory has always been the multiplayer. Call of Duty 2 was incredible, and 3 was on par albeit slightly less fluid compared to its predecessor. It goes without saying that Call of Duty 4 is hands-down the best multiplayer experience I've ever had! Everything is brilliantly thought out and the levels are designed fantastically. There are so many features I want to talk about!
Perks: Everyone knows about these from the beta, but there's some confusion to the levelling and over-powering that perks give you...Basically, they don't. Perks are decent, and useful, but they are not overpowering whatsoever. Yes, there's a perk to shoot through walls, but only if the walls are pretty thin and made of a lighter material. So you can't shoot through thick building walls or anything.
Another important aspect of the 'perks' is that you only get the very best ones when your rank is very high, and that takes a long time to achieve. There's a ranks system in COD4s multiplayer where you get promoted based on EXP gained from killing opponents, meeting challenge requirements etc.. and it's well balanced to take people a long time before they get the best weapons. And by best, I mean different. Fundamentally the weapons have similar stats, it's just having the prestige of using fancy weapons over bog standard ones.
But yeah, as you can probably tell by now, I think the game is very good. I'm also 100% certain that my thoughts are echoed across many of the thousands of current online players out there. Get this game, you'll lose your life, but by God is this a worthwhile game to lose it to!