One of the best First person Shooters I've played in years.
You start out on your basic tutorial course where you're taught to shoot and what buttons do what then you have the option to run the course again and try and beat the games (and on the 360 at least, the developers) personal best.
For me it was challenging but not ridiculous to such an extent that it made me stop playing or give up entirely. That's how the entire game feels to me personally. I completed the game on Regular (Normal) Difficulty and there were times where I was shouting at the TV because a rogue grenade took me out (you will encounter this a lot) but it didn't feel as I was being cheated, it was true to life and realistic. If you stand near a grenade and it goes off you're not going to be able to shrug it off and carry on shooting at the nearest bad guy, you'll be seriously wounded if not dead.
The SAS missions are more about espionage and silent approach early on whilst the Marine missions throw you in to the thick of it a lot quicker, the sheer amount of enemies that come flying at you at certain choke points in some missions is jaw dropping but whilst it does make it a bit more tricky it makes it all more worthwhile when you take them down and complete the mission.
The controls have had a little re-think and the crouch button for the xbox 360 controller is B instead of the generic left/right stick "button" (R3/L3 if you like) the L3 is used to sprint now which is very handy when you need it. You've still got options to throw grenades back and alike but the B button as crouch is certainly a turn for the better.
The level design of COD 4 is superb with graphics being crisp, subttle and true to life. The mission "All Ghillied up" where you have to play a sniper in Ceynoble is abosutely amazing, the entire time I played that mission I was in awe of everything around me, the surroundings, the atmosphere and the story development it was truly entertaining.
As soon as I received this I played this game for about 8 hours straight then finished the game on the day after. It is a bit short but with the levels and story that you are presented with, it'd be practically impossible to feel hard done by.