Aiming was super intuitive and I got that down pretty quickly. The thing I had more trouble with is movement. I think controllers have that down way better. I also had a little bit of trouble using all the other keys that stuff were bound to and then returning my fingers to WASD. I'd always have to look down and that in turn would get me killed. It's gone away with practice but I still prefer movement with an analog stick.
Interesting. I'm the total opposite. I've grown accustomed to the key bindings (part of it is I'm very familiar with a keyboard and can type at 120 WPM consistantly, I've taken many tests). It's the mouse sensitivity that makes it difficult. I play with a very high sensitivity because I'd rather get used to that now then try and get used to it later.