Infinity Ward took a gamble moving into Modern Warfare and it paid off!
The main story mode is centered around S.A.S. and US Marines operations primarily in the Middle East and is an enjoyable experience. There are many melodramatic moments in the story, but they work really well with the plot and immerse you into the game.
The combat is very hectic and requires the use of tactical shooting (running for cover and looking through your sight before shooting) if you are looking to survive. The AI is an improvement from the earlier installments of COD as well. Not only are your enemies smarter and more resourceful, your teammates are also very useful - not to the point where you feel useless, but they dispose of their fair share of bad guys!
The weaponry in the game has been re-created from real life with great accuracy, from the side-arms to the gun-ships. The maps are beautifully rich in detail and sound is amazing.
There are two drawbacks with the single-player mode. Firstly, it is too short (about 6 hours long) and a co-op mode is missing, but I'm just nit-picking. Otherwise it is a great experience, and after it, you have multi-player online to look forward to!
The online mode employs a class-based system with experience points (and levels) gained by killing, assisting and completing challenges. You are able to create your own classes and choose your own weapons and 'perks', which are extra abilities which aid you during battle. These perks range from more health (Juggernaut) to dropping live grenades when you die (Martyrdom) and are gradually unlocked as you level-up. From my own experience I can say there is minimal lag issues and the game runs extremely smoothly online.
All in all, COD4 is a superb game and is as near to a must-own game on the PS3 as anything else thus far. The single-player mode is fairly short but is a great shooter experience and the multi-player mode is very addictive - it's well worth the £40!