I originally played on consoles so a controller feels more natural for me. I've spent unhealthy hours in FPS games over the many, many years. :-D
CS GO is no different, and the buttons that can't be assigned (the Z,X,C chats and the U,Y chats) are barely used, in my opinion.
I can buy guns and get out the gate quicker than most, plus my movement is much better and smoother.

I feel bad for people who think CS GO is some elite game that can only be played with a keyboard and mouse, it's not. Don't listen to the advice of someone who's used to keyboard/mouse, then decides to try a controller out. Take the advice of someone used to a controller using a controller.
Each to their own & all that :-)