Truly the unique selling proposition of the X100 is the hybrid optical and electronic screen. There are 3 modes - 1) Pure optical, 2) Optical with electronic overlay for frame lines and information, and 3) Electronic viewfinder with information. I have found that I use either 2) or 3); I use the Optical viewfinder with framelines when I am outside taking shots, and the Electronic viewfinder for inside, or when shooting anything closer than 3 or 4 feet away.
It really is a great feature and it makes the camera a joy to use.
The screen can also be switched off, work as a viewfinder or display information; all useful modes depending on what you are going. The screen itself is nice, but as it doesn't tilt, is slightly lower resolution and slightly smaller in size when compared to the Sony NEX, it isn't quite as good. Usefully the screen switches off when you put your eye to the viewfinder.