The only thing I missed when I switched from a DSLR to my X100S was a longer lens for portraits. So when this converter was announced by Fuji I knew I had to get it. What it does is to turn the 35mm (equivalent) lens of the X100S into a 50mm. After using it for a couple of shoots I can say that this is definitely a quality piece of glass. I don’t pixel peep but I haven’t noticed any loss in sharpness over the native lens.
I shoot mainly portraits and can’t afford to have a second system with interchangeable lenses, so for me this converter was a given. However, the quality comes at a pretty hefty price for a simple converter, so I wouldn’t recommend it to everyone. If you know you want to shoot tighter portraits or if you have some other specific need for a longer focal length, then go right ahead and get it, you won’t be disappointed. On the other hand, if you mostly shoot general stuff and you’re fine with the native 35mm, then I don’t see any reason to spend money on this.