Here were my initial impressions after I got my S5 (after using a D80):
+ Amazing build quality and ergonomics (well it is basically a D200), it's also very snappy and responsive in terms of general operation.
+ Produces amazingly good jpegs out of the camera
+ The huge dynamic range is very useful outdoors
+ The shutter release is so much nicer than my D80 - it's hard to explain, but my D80 felt like a click, as opposed to a nice squeeze on my S5, so it feels a lot like my Nikon F3 in that respect.
+ The tones that come out of this thing are very nice, as is the lower amount of noise.
- Menu isn't as intuitive as Nikon's - for example the image review, which just feels clunky - on the Nikon you have two buttons dedicated for zooming into/out of the photo, the d-pad is dedicated to panning and the command dial is used to switch between photos. On the Fuji, pressing up/down on the pad controls zooming - to pan you need to press another button to switch the d-pad to panning mode. You also can't use the command dial to switch photos, you have to use the d-pad, but only if you're at 0x zoom. Luckily I don't really use image review that often, it isn't unusable, just nowhere near as well thought out as Nikon's menu system.
- RAW files are huge (25mb!), and Adobe hasn't made any proper profiles for Fuji's RAF format, so if I want to apply Fuji's film simulation modes I have to use Fuji's super-slow software.
- As a drawback to the huge dynamic range (which you can adjust from 100% to 400%), the FPS takes a hit. This doesn't bother me though as even when I shoot sports or any action I tend to wait and get the timing just right.
Those impressions still stand two years later. Bear in mind that you need sharp lenses to make the most out of the Fuji as its anti-aliasing filter is pretty strong. Noise isn't an issue provided you nail the exposure.
Here's an example shot:
ISO1250, Nikon 18-135mm, Straight out of camera jpeg