X100 AF only sees vertical lines
My friend, who already shot thousands of frames with his X100, noticed some interesting AF behavior. The AF does not lock on horizontal lines. It only "sees" vertical lines. You can test it yourself if you have a kitchen or bathroom with tiles on the wall. I didn't notice it at first when using my X100, because I'm already used to using vertical lines for focusing - as I shoot a lot with manual cameras that have a horizontal split-screen indicator or rangefinder.
So if you have problems with X100 AF this might be interesting for you. Simply imagine you had a horizontally moving RF patch and concentrate on vertical "features" of your subject.