...Anything you can't do with the fixed lens?
The X100 lens should not be used for close up work work at apertures wide than F 4. The X100 manuals state the lens was not designed for this purpose. At F 4 and above, close ups are fine.
By the way, the FUJIFILM Macro Mode does not affect lens optics. Macro Mode just switches to an AF algorithm that is optimized for focusing on close objects. This algorithm is unsuitable for non-close up focusing.
The 100 lens is reported to be unsuitable for IR photography because of IR hot spots.
Using lens filters require an adapter.
I find the X100 lens renders differently below ~F 4 compared to narrower apertures. At wider apertures there is more flare and risk of internal reflections. The good news is you have a lens with dual rendering characteristics. The flare below F 4 produces a lower contrast image. at F5.6 and above the contrast resembles the better XF lenses. The X100 lens is quite long. Most of the lens is inside the camera body. At one time the X100 rear element to sensor cover glass distance was the shortest of all digital cameras. This is responsible for the dual rendering behavior.
I think this
review of the lens is fair.
Any reason it couldn't be your only digital camera?
Yes.
o The leaf shutter is limited to 1/4000 sec.
o The fixed focal length is limiting. Using the teleconverters cancel the camera's primary advantage of being light and compact.
o The controls are small and cramped - if you constantly need to change operational modes, this is tedious.
o The X100 is not water resistant.
I own a X100T and a X-Pro 2. For me, the X100T is a great daily carry camera. It is also good for candid work. I find a people are less likely to be concerned when I use a chrome X100. The X-Pro 2 is more flexible when I can't zoom with my feet and is easier to use.