Glad to see someone started the discussion on the XPro1.
Anyways, my 2 cents: this camera is interesting in the way that what this baby could accomplish still has to be seen. IMHO, this camera is a "system" by itself - not a stand alone like the G-series camera (you don't need to buy into a lens system with G12 or G1X).
Now, I think the question what most people would be interested is what the XPro1 is capable of. While there's no question in terms of aesthetics (I think it's beautiful), I would question whether I can capture - say, a landing F-16 or the split second when the batter hits the ball - with the XPro1. In my case, if it cannot do that, I have no choice but to stick with my current camera to carry out my assignment.
Whether this camera can serve as a back up or not, it really depends on your personal need. For me, even the X100 is too bulky to serve as a backup. Sure, I try to bring it with my other gears when I go on assignments, but frankly I think a small-size DC (like the S95) or a EOS body (say 600D) would be more ideal than the X100.
I can probably say the same for the XPro1, since it's not going to be any less bulky than the X100.
So what's the X100 doing in my gear pack? Well, when I'm in the restaurant and find something interesting but doesn't want to be the center of attention by pulling out the pro DSLR with the big lens, I use my X100. It's good to remind myself that I can take a photo without all the fanfare (sure, iPhone works too, but I think the X100 looks more stylish
If my assumption with the XPro1 is correct (based on what's out there - press release, interviews with Fuji staff, etc.) , it probably can't rival even the 7D in terms of speed. Quality wise? Probably not enough to threaten the FF 5D2.
However, I think the value of Fuji's RF-look alike system is that it's something interesting for the shooter who wants something different - a body that's non intrusive, still offers good quality shots, able to change lens, and - even better - a Leica look-alike that has a great OPTICAL view finder which can also work as EVF when necessary.
... sorry, I had to get that out. Seriously, Olympus had a great idea with the PEN series, but it seems their engineer has forgotten that sometimes we like to look into a viewfinder w/o having to turn on the power (or is it just me?). Now at least the X-series camera is doing it right (except for the x10 - that VF is just too small).
So for me... the XPro1 is definitely not for work, since it's going to be tougher trying to capture split seconds with primes compared to zoom. However, I am going to get the camera for my own enjoyment (and it'll probably see a lot of use in that department - casual trips, dinners, etc.)
While I'm sure Fuji has some tricks up their sleeves, I'm still intrigued by how the VF of XPro1 is going to work with the manual focus lenses design for M mounts (especially focus and light reading).