This is probably the single greatest modern warfare game out there.
Note: I do not hate Halo 3 and/or the Halo franchise. I use it simply as a comparison. Thank You.
With a slew of WWII shooters still coming out, it was finally time for one of the kingpins of the genre to step out of 1944 and move away from the overused forests of France. Infinity Ward's Call of Duty franchise was the first to catch on to this, and in doing so they came out with one of the best shooters of all time.
The graphics in Call of Duty 4 for the PC are extremely beautiful, especially if you have a PC that can handle the higher-ends of the graphics. Certain settings do not do this game justice, but overall the game absolutely shines. All of the weapons look beautiful in the game. Setting up the game at higher resolutions on smaller screens makes the font too difficult to read however, which can be a problem for some people.
All of the sounds in Call of Duty 4 are done very well also. The guns sound like themselves and I like the fact that the M16A4 and the M4 Carbine do not sound like the master of weapons like in most FPS games. In fact, I believe the most brutal sounding weapon is either the Barrett M82 (.50 Cal) or the Desert Eagle. Using said weapons feels easy enough to do, the controls aren't clunky; although I really wish that the melee button wasn't the V button sometimes.
The single player game is one of the best in the Call of Duty series, but it's also one of the shortest. There were many missions available in the game, but they were also quite short. The lack of quantity is definitely made up the by the quality however, featuring some of the best missions in an FPS game that I have ever seen. Some of the missions near the end of the game are so good that I replayed a few of them about 5-6 times just for the hell of it.
The online multiplayer for COD4 is amazing, even though it's over a year old at this point. You have 5 classes that you unlock, and 5 slots for custom classes. Each custom class has a primary weapon with attachment, secondary with attachment, primary grenade, secondary grenade, and 3 Perk slots. Perks are ability enhancers that help out with whatever play style you are. They are all put together well enough to where no one class feels too powerful or overbearing. Matches are easily found, with 13.5k severs now; but I noticed a major lack in game types other than Team Deathmatch, although lately I have seen a good amount of S&D going around.
Call of Duty 4: Modern Warfare is still an amazing game despite it's age in the modern gaming world. The population is still hitting the millions and there is a server for everybody to play on. So if you have the $35 to get it on Steam while it's still on sale, go do it. It's worth the money. You're practically buying yourself fun.