I'm a Fuji S3 user, about to jump to an S5.
The S3 is built into a modified Nikon F80 film camera which has an inbuilt grip much like a Canon 1D. The back protrudes a long way out past the original outline of the F80 as well so it's a very chunky camera and heavy, but it's so comfortable in the hand and stable too (I can hand hold it at lower speeds than any other camera I've ever handled including ones with stabilisation!)
The sensor and associated gubbins are entirely Fuji, there is nothing Nikon about it apart from the bodywork.
The sensor is a twin 6MP Super CCD SR unit, which when shooting raw gives you unparalleled dynamic range in highlights. Using ACR I've successfully managed to pull back over 4 stops from blown highlights, though the further they are pushed the more likely they are to take on some wacky colour - mine go shocking pink!
In action this is a painfully slow camera to use - forget it for anything where you are going to fire off more than 3 shots at a time. It runs at approx 1fps to fastish CF cards, and will take another 30seconds to a minute to recover before you can shoot anything else. This performance gets worse with XD cards, and gets worse as batteries lose charge.
Speaking of batteries it takes 4 AAs which must be rechargeable - it will not switch on if alkaline are fitted.
If you want outstanding colours, excellent highlight detail and don't need to be pixel peeping or machine gunning the shutter you will love this camera.
If you need very high ISO, machine gun shutter or the utmost in fine details and sharpness forget it.
I personally love mine, but the size of the thing means I can't fit as much gear in my bag as I want to take with me, so I'm going to shift to an S5 which is the same body as a D200 (but with the same Fuji innards as the S3)