I am using this Flash with a Fuji X T20 and it works extremely well. TTL is dead on either in direct or bounce flash mode.
Now, if I was still working as a Professional Wedding Photographer, I don't know if it would be my first choice as there a so many great units available.
For general usage, it works very well and is not overpowering weight wise on the smaller mirrorless cameras in vogue today. One nice addition to its kit is the pouch that can be attached to a belt so the flash is always available.
Also, if anyone is planning to move to mirrorless, the XT 20 is a terrific camera, particularly when combined with the 18-55 f 2.8-4 lens.
Compact and right size for a mirrorless camera (Fujifilm). The flash recycle time is decent and don't expect it perform on an ultra fast burst mode. Sadly, no HSS (high speed sync). Does everything you expect a flash to do but in a much smaller package. If only having a Li-ion rechargable battery pack would have made it perfect instead of 4 AA batteries. Buying on a full price of 269$ is way too much and I don't think it is worth it.
I've been getting inconsistent results with its use on my X-T1 but there is a learning curve for its use. This is the best instructional piece I've read on its use with the X-T1... search for johncaz's blog on the nissin-i40-flash-for-fuji
I wish Fuji would make flash use easier on their X cameras.