Took this based on my Amazon review of the EF-42
This is a basic budget flash gun and OK. It has has a "plasticy" feel to it and the battery door is fragile so needs care when opening and closing. It's certainly isn't a Professional grade flash unit, but your not paying enough for one of them anyway. In my opinion Nikon has the best flash system around. I worked as a pro, for many years, using Canon, Olympus, Pentax and Nikon Flash systems and base my opinion on working experience with these systems. This might not be fair, but if Fuji expect to get back into the pro market again they'll have to up their flash system. Their old Pro DSLR's were basically Nikons so they used their flashes.
It works well on my Fuji X-E1 giving me constantly reliable exposures. Recycling times with Sanyo eneloop batteries are better than I expected, but I've been spoilt by add on power supplies.. It also works of camera with a third party off camera cable. You need a OC-E3 type Canon fitting cord. There is an extra contact not used in the case of non Canon cameras. No problem with attaching it to the camera or the cord, saw someone had problems with this. A Nikon SB 600 stofen type diffuser fits
Top sync speed is pretty low at 180th of a second on the X-E, so outdoor use for fill flash could be a problem. Check your camera specs.
In auto the camera sets a very low sync speed in low light. To overcome this I set the shutter speed to a manual setting. This can mean you need to set the Preview exposure preview in manual mode to off, otherwise you viewfinder will probably look dark, in low light, as you are using it to show the exposure. This is on the X-E1, maybe the same on the X-Pro1 and is found in Set Up menu 3 on the X-E1
This isn't a sophisticated flash and is designed for basic use, which it does well
Hope this helps
David