Amazing multi-player experience, Very Difficult single player if on the proper difficulty.
The key feature, just as it was in Call of Duty 4, is the multiplayer experience that you are able to have. With all the upgrades and prestige options it will have you sucked in, in no time trying to get to that next level or unlock that next achievements to get you new perks or new weapons.
The single player when playing in regular is challenging and has a solid story that drives it but the true challenge is when you up the difficulty and try to take on the massive amounts of enemies while only being able to take a shot or two before you are killed. The end results of you beating this single player campaign is the enablement of the zombie mode which also adds to the value of this game. The zombie mode is also challenging and when you take it online with co operative play it really hits home.
This game may not be revolutionary in any sense due to it being a mirror image of its predicesor call of duty 4, but they still give you something to do while you wait for infinity wards version that comes up the year after.