Interestingly, Fuji has just released their WCL-X100 wide angle converter lens for the Fuji X100. This 0.8x converter turns the X100's 23mm lens into a 18mm lens, equivalent of just under 28mm in Full Frame terminology. One thing about this converter is that it looks extremely well made and well designed, unlike those cheap and nasty converters that every small New York photo shop seems to want to sell you. It features multicoating (Super Ebc in Fuji speak), has four elements in three groups for proper correction and sharpness and is meant to be extremely well made. This converter attaches by screwing into the 49mm filter thread.
At $349, it sure isn't cheap, though compared to other high end converters such as Nikon's TC-20 II 2x teleconverter, it is priced similarly.
Early reviews suggest that the image quality is excellent, almost as good as the 23mm lens on its own. There's meant to be some barrel distortion that is corrected when accessing the appropriate setting on the camera, which I believe requires the latest firmware; but that's about it.
The major issue that I see is that 28mm is a bit close to 35mm in focal length. Maybe it would have been better to go to a 0.7x converter which would mean a focal length of approx 15.5mm, or a full frame equivalent of just under 24mm. Of course this would result in more design challenges, and therefore purchase expense, to retain the top notch quality that Fuji was requiring.
I must also say that on my Nikon SLRs, the further I move away from 35-50mm focal length range, the less I use the lens. My fisheye and ultra wideangles don't get much use, and I sold my 80-400mm for lack of use. So, maybe I would get more use out of a 0.8x converter than a 0.7x converter.