So recently I purchased Call of Duty 4 based upon the reviews of Tycho and my friend MatchesMMalone. Let me say that the Multiplayer mode had me hooked right away. The role playing game aspects of experience leading to new abilities and new weapons is really addicting as the leveling up is not tedious in the early levels and really addicting as you bring new weapons into play. Basically as you progress through the multiplayer game you have experience affecting your rank and personal abilities, along with unlocking new sets of challenges. Challenges are ways of increasing rank more quickly as you get experience bonuses for accomplishing a challenge. One challenge would be to kill 5 enemies while in the crouched position or perhaps to call in 5 UAVs. Then there are also specific challenges associated with each weapon in the game. By completing these challenges, you not only get experience but you can acquire new camouflage for the guns or new scopes and capabilities.
So multiplayer aside, there is a great single player campaign here as well, but it is missing a multiplayer coop mode that would have made this absolutely stellar. The single player campaign puts you in the shoes of 3 separate soldiers with specific objectives to meet. The missions are well scripted (much to the same degree that the missions in Halo 3 or other Call of Duty games are well scripted), the story is well told and missions range from easy to fun to challenging without getting to terribly annoying. I ran through the game on Hardened difficulty and really got stuck in only a couple of places where I spent ungodly amounts of reloads attempting to accomplish the mission. But the ending is satisfying and spectacular and very anti-climatic.
So based on this, the multiplayer aspects alone would warrant getting this game, but add in a fun, exciting (~6 hours) single player mode and I think we have a winner.