matteo

I´ve had mine for a week now.   It seems to be the lens we were all hoping for, it's very sharp wide open at 400mm and focus speed and stabilization seem good.  I had the Canon 400/5.6 L before and the 500/4.5 L.  The 500mm lens is the best lens I've ever had and worked great with the 1DsII I had at the time.  The X-T1 + the XF100-400 may not be there but  getting closer and hopefully the X-T2 will have improved tracking capabilities.  Overall I'm very pleased with the lens.
 
Here's a test shot with the X-Pro2 (pre-production model) + XF 100-400.  Lame subject ...but great feather detail.

 
I also think it looks quite nice on my GS X-T1.

Some more examples (click to enlarge):
 
Cardinal
 

 
CAMERA Fuji X-T1 LENS XF100-400mmF4.5-5.6 R LM OIS WR ISO 400 FOCAL LENGTH 400.0 mm (600.0 mm in 35mm) APERTURE f/9 EXPOSURE TIME 0.002s (1/500)   ----------------------------------------------------------------   Hawk in flight     CAMERA Fuji X-T1 LENS XF100-400mmF4.5-5.6 R LM OIS WR ISO 200 FOCAL LENGTH 400.0 mm (600.0 mm in 35mm) APERTURE f/10 EXPOSURE TIME 0.00222s (1/450)   ----------------------------------------------------------------  
Red Squirrel
 

 
CAMERA Fuji X-T1 LENS XF100-400mmF4.5-5.6 R LM OIS WR ISO 200 FOCAL LENGTH 400.0 mm (600.0 mm in 35mm) APERTURE f/6.4 EXPOSURE TIME 0.00286s (1/350)

I still like the lens, and I still like the background it produces:
 

 
(it's ISO 2500 btw.)

Mushroom Picker II
... in Monti Sibillini/Italy (Variation with the 55-200)