What makes someone using a mouse and a keyboard so much better? Do they somehow have better aim than me? Do their grenades somehow do more damage? Do their bullets somehow hurt me more? Does the keyboard somehow magically let them know what I'm about to do before I do it?
I have yet to receive an answer to this no matter who I ask.
I'm the biggest fan of FPS, I've played them on consoles, on PCs, and I've played all the biggest names.
I've personally studied this topic religiously, so although I hate to toot my own horn, you'll probably get the best answer from me.
Let me tell you that first of all, if I have the choice to pick any type of controller or keyboard, I would choose a wired xbox 360 controller (there is a HUGE difference between a wireless and wired, so that is very important).
Keeping that in mind, for gaming especially FPS the Keyboard and Mouse crush the controller.
There are two main reasons for this.
The first is that most if not all people are taught how to use mouse and keyboard very early on, so they are already comfortable with the controller before they've ever played a video game before.
The second is that think about the anolog stick you look with on your controller. Think about every single position you could put the stick in. Now think about a mouse. The amount of different positions for the anolog controller will be a fraction of the different positions you can acquire with a mouse. What does that mean?
That means that lets say you are playing Call of Duty 4 (a game that is both PC and console). Same situation, one on the console, one on the PC. There is a guy behind you who just shot and missed, no matter what his next bullet is going to kill you (you know this because your a psychic). The only way you can kill him is by spinning around and shooting him right in the head. Simply said, the percentage of people that will be able to accomplish this on the console, will be a fraction of the people who will be able to accomplish that on the PC.
When it comes down to it, they both have their advantages and disadvantages. If you use an XBOX 360 controller on a PC game, you are going to have an advantages over the Keyboard users, but their advantage is much more significant. FPS are all about accuracy, Keyboard and Mouse make it the easiest it can be.
I've played through well over half my ranks in the PC game: Battlefield 2 with a wired Xbox 360 controller, and I can tell you although the difference wasn't huge, there was countless scenarios where I knew if I had the accuracy of a mouse I would've survived.
Personally I prefer the wired xbox 360 controller, and I prefer console games, because I've personally noticed that there are a lot more people that svck at using a controller, than people who svck at using a mouse and keyboard.