PC gamers who build their own systems are into the tech side of things in a way console gamers are not. I am presently midstream in my first custom build, and am enjoying the learning curve. I am not only doing the research to make good component choices, but also trying to gain a bit of an understanding of how all this hardware works, which is a rewarding challenge.
I agree that the mouse and keyboard are a superior approach at least for FPS games--I have also played them on XBOX. Also, I like the experience of having one machine that does so much--surfing the net, high end graphics rendering(in my case with Photoshop--I am a visual artist), downloading, movies, music, etc.--and a high end PC built for gaming does all of this faster and better than a cheaper one.
Delving into the whole tech side of things, which becomes a necessity for system builders goes beyond just gaming, and involves a very stimulating learning experience.
I ahave been playing HL2 Deathmatch (multiplayer online) for some time now--there are constantly really cool new maps being added to the servers, and so the game truly remains very fresh. Also PC game mods for single player give ever new content keeping old games alive.
Generally speaking, I would suggest that PC games at their best are just higher quality--better graphics, better physics, etc.
Anyways, when you were bored at work you started researching sys tem building--obviously that was more interesting than reading about the latest console news. My machine is being built with gaming in mind, but it also does so much more including serious work related tasks, so for me the cost is worth it.