I've been using the X Pro alongside my Nikons for a while now (a bit of a mistake as it turns out) but the other week I jumped ship and shot a 9 hour event in London on the X Pro and the X-E1 with the 18mm on the little XE and the 35mm on the X Pro. 95% of the shoot was by available light and it worked really well.
Having just the X camera interface to concentrate on was a big jump for me. Prior to that I'd been juggling Nikon and Fuji and not using the Fuji's to their full advantage.
The 24-70 f2.8 on a D3 was my Nikon workhorse and the two little Fuji's substituted admirably. The lack of weight was huge for me - all day in London with two DSLR's + lenses is no fun.
I got a much better reaction from the 120 strong party I was photographing, it's obvious in hindsight really as the Nikon viewfinders are centrally mounted so the cameras cover most of your face.
With the corner mounted Fuji viewfinders the subject(s) can see most of your face and able to relate better. YMMV but this was something I noticed. Lots of remarks about my "old cameras' as well, which made me laugh as I'd picked up the X-E1 two days previously.
All in all, the X cameras are great for social photography IMO. Not so hot when I really need a 70-200 f2.8 but the rest of the time, just great. The process of photography (undeniably slower) is just more fun with the Fuji's.
I found the meters to be better than my D3 which meant that post production was a breeze - I whipped through the excellent jpegs (augmented by the superb auto white balance) in no time.
HTH.
