Whilst I wouldn't agree with @Crazycrab's brazen way if expressing them, I do actually agree with with his points. I almost never use M/K for game control on PC, with the exception of strategy games, some MMO's and some top-down RPG's. I don't even like using M/K for shooters despite the higher accuracy potential, it just doesn't feel as fun to play for me. I just feel more of a connection to a game holding a controller in my hand. Yes, I'm not as competitive that way, but I get round that problem buy not playing competitive FPS games on PC, only on console where I know it's a level playing field. Simple.
The fundamental difference between a game controller the M/K concept is that one was created specifically to control a video game, the other one wasn't. M/K has come a long way, but hasn't fundementally changed. That is why M/K douse base problems with certain games and gameplay mechanics, like stealth a @Crazycrabsaid. It's that way because it still has to serve its original intended purpose. As for game controller ms? They have evolved a lot. We aren't playing with Atari Joysticks anymore, that's for sure. Controllers have become almost unrecognisable from what they used to be and are far more sophisticated and advanced, and have gone though this evolution at each step to improve one's connection to the game. That's just my own little theory as why I personally just enjoy a a controller alot more that a M/K.
Not that M/K doesn't have its advantages as well. It's precise and offers you're hands and arms more rest as you play. As I said, the it's best way to play games where you manage as opposed to taking direct control, like RTS. It's also great for MOBA's and certain MMO's that uses a large array of control options. For any game that requires any even a remotely substantial amount of typing, a controller is a pain the ass, so there's that to.
However, in the end, it's not about one being better than the other, it's about how each individual feels is better for themselves to control and connect to the game. Now @m76, I'm gonna be honest with, you I do believe you look down on people for using a controller on PC, and that's not fair. It doesn't affect you how another person plays Skyrim or whatever is on PC. You prefer M/K, not a problem, and this evening I'll probably be playing my modded version if Fallout 4 with a controller and I'm sure we'll both sleep just fine.
One last thing to all of you, keep it civil. That is all.