This thing is pretty sharp and fairly fast. The OIS is also excellent. I am taking photos at night down to 1/15th without a problem. I am having better luck with this than any of my faster non-stabilized lenses, which I guess makes sense. However, it feels better because in order to work at these slow speeds, the ISO has to hold up, which it does in the X-T1 compared to my Nikon D7000.
The build quality is great. I don't really like the thin channels on the rings. Don't get me wrong, they feel good and work well, but tiny bits of gunk get in there and are a pain to get out for someone that is OCD like I am!
Out of the 5 Nikon lenses I run with, only one is variable aperture. Although I thought the f/2.8-4 would bother me, it really doesn't. I may lose a bit of DoF, but I don't have to squeeze every bit of light to get a good photo. Next lenses will be the 55-200mm f/3.5-4.8 and the one I'm really drooling over, the 56mm f/1.2
I will have to say, I'm a little weirded out by the aperture ring not being marked and having stops at either end, but I'm living with it. Yet another thing to get used to.