I mostly play with keyboard + mouse to save me the hassle of setting the controller properly (not all games can handle it well), unless I run into glaring complications while using them, like with Dark Souls. Of course, as plenty of people have pointed out, some game genres are unplayable with just a keyboard and a mouse, like racing games (for the love of God, don't even try that), and the same is true for the opposite. However, I don't think shooters are that non-viable with controllers. They are harder to play, sure, but people have been playing FPSs on consoles for at least the past 10 years, and I don't believe them all to be insane.