Got my FM2015 a few days ago, so here is my opinion:
Build quality: I expected worse. it is put together quite well, but mine has a few scratches right out of the box, but nothing to worry about.
Buttons/Scroll: Buttons are nice and light, pretty good feedback compared to a deathadder and the scroll wheel is very smooth with very light notches, but the click is kinda hard on it. The sidebuttons have a weird shape, but overall it feels good.
Sensor/performance: Excellent and good tracking. good glide on the mousepad aswell due to its light weight.
Shape: Now here comes the main problem: i simply cannot hold the mouse naturally. I have 20.5 cm hands and i am using claw grip. my ringfinger and pinky curl up quite a bit and on the mouse there is this ledge on the right side pressing in on my ring finger, wich hurts quite a bit.
so i guess i'll be waiting for an ambi version and using my ZA11 till then