If you liked Call Of Duty 4: Modern Warfare, then you will surely love this game. I won't lie, it is basically the same just set in a different era. There is still that fast paced "arcade" style feeling to it, which is what everyone seems to enjoy. That was my only problem with "Call Of Duty 4" (I used to not be able to keep up, and would constantly get killed)
I was positively surprised with the increase of challenges and unlockables. CoD 4 had a lot, but this one added a pistol marksman challenge, and several other challenges. It's great fun. Not to mention there are now 65 available ranks, including the previous addition of Prestige mode.
If you asked me which was better between this and 4, I couldn't give you a solid answer. They are both pretty amazing first person shooters. I recommend buying CoD: World At War.
9/10