I just got the FK about a week ago and it has been a huge change from the g9 I had been using for the passed few years. It is definitely much smaller and trying to use a palm grip results in dragging my thumb. The biggest learning curve is making sure my thumb and pinkie are bent inwards so they don't drag. This is a great and comfortable claw grip mouse and my aim has definitely improved over what it was with my g9. It has just been very unforgiving if I ever fall out of claw for whatever reason. Other than that I love this mouse. I haven't hit a button I didn't mean to, and the scroll wheel is very precise (to the point that when my keyboard was fucking up I used to switch weapons and it worked pretty well). Oh, and other thing that takes some getting used to is the lift off distance. Basically it will only move the cursor if it is touching the mouse pad, which is great but I have fucked up a few times for not compensating properly. That's just a matter of feel I'm sure anyone would pickup over time.
*I may have clicked too hard at some point and now the left click sometimes squeaks a little on the way up. So now I have a mouse that squeaks.