The EF-42 is fully compatible with X-Pro 1. It also works with the X100 I have and the X-20 that also my father (but this is too big - much like Nikon SB800 and a Coolpix P5000 or 7000).
The production is not comparable to Nikon, it is bine more plastic and less "pro", but knowing the Fuji variety and why the EF-42 was put on the market, it is not a concern. The important thing for my father (and month) was to finally have a powerful flash and rotating cobra for the X-Pro 1.
The EF-42 did his job perfectly, very good balance even in direct light. Quality Nikon might say.
On the X-Pro 1, the size of the EF-42 does not feel (I tried) and manipulation of the X-Pro 1 hooked up with is very pleasant. There is no manifeste.De instability thus the EF-42 is the essential addition for one who seeks a cobra flash for its X-Pro 1 or the EX-1.
For X100 / X100s I much prefer the EF-20 remains very compact and swivel upward. My father and I do not recommend the use of the EF-X20 malgrés its construction, its compactness and its use by 2 AAA batteries instead of AA.
Noted as the EF-20 EF-42 does not provide power management for plaice. So pay attention to rechargeable batteries and still have a drumming in reserve. Expected version X breathable product a little resistance and pro (in relation to X-Pro) and energy management. Hence the loss of a star, but offset by the excellent price at which Amazon sells it and that the EF-42 has the merit to exist and be compatible with the range X.