Me, always turn off.
Motion Blur only works best for 30fps and since PC is always at least 60fps or higher, it isn't needed on PC. I tried it at both 60fps and 30fps and it looks great for 30fps but unecessary for 60fps. Motion blur is for consoles since they run at 30fps and it masks the awful visuals a bit which it helps. But at 60fps and above, it's just painful.
I know gamers have different taste and so with that, do you use Motion Blur?