I shot with the X1D in studio with a model about 3 months ago during NYC Fotoworks and I was not impressed in the least.
The autofocus was horrible and inaccurate at least 50% of the time. Additionally, there is about a half second delay in the view finder, which makes any kind of serious handheld work impossible.
A very good lifestyle shooter once told me, his commodities are expressions and expressions are fleeting commodities. If you need to capture that perfect expression, by the time you see it in the view finder, it will already be gone.
I tested the X1D against the H6D and found that to be much better, especially when using true focus.
Also, if you switch to manual focus, the view finder automatically zooms 100% to the center of the frame, which make composing your shot kind of hard. By the time you nail focus, then stop moving the focusing ring, wait for the view finder to return to full frame, and recompose your focus will probably be off due to you or the subject moving.
Personally, I think this could be a very nice feature if they programmed it to just zoom to 100% in the center 1/3 of the view finder. Then you could accurately focus and still know what your frame is.
Ergonomically, the camera felt really good in hand. I also like the the lighter weight, however those do not make up for what I listed above.
All the other pros who attended NYC Fotoworks, which there were about 50 to 60, had the same thoughts. Ot at least all those I spoke to, which was a lot.
As a professional, I just will never be able to deal with any kind of delay in the view finder for handheld work. It is a deal breaker!
I know the latest fad are EVFs, but they still do not render the view better then a prism and your own eye, especially in low light or high contrast scenes. (Remember, the sensor is interpreting the view and making adjustments based on how it is programmed, then communicating it to the EVF. The problem though is the sensor has a significantly lower DR then the human eye, which does not work with contrasty scenes and is very different then a mirror and prism.)
The only advantage they have for now is they allow for a smaller camera. Not to say that won't change.
Recently I bought an XF system and I am using it for construction documentation of LGA. I really like the camera.
It is heavier then a 35mm, but with primes lenses nothing that is too cumbersome. On the days that I am at La Gaurdia, we are on our feet for about 10 hours (with a break for lunch of course). I have yet to feel weighed down due to the camera.
Also, Phase did a great job of damping the mirror shake. With some practice, I am able to get sharp images at a 1/50 with a 110 mm. If I turn on the mirror delay, I can get to a 1/30.