I am a bit late to the party, but I finally collected my housing last week and dove it at the weekend. I have been using the Nauticam NA-D4 for much of the year, plus some brief liaisons in Egypt and Mexico with the D800 (also in a Nauticam). So it is nice to have my own housing again (that I can customise a bit), rather than borrowed gear.
As the eagled eyed will notice, I have three flash synch's set up. One is really a spare in case of a water damage in the field. This will probably be replaced with a different port when some of Subal's accessories come online. But at present they are working only on housings - dealing with the backlog of orders.
I also have a vacuum seal on my housing. The D4 is expensive, so I went for belt and braces.
One of the most obvious new features is the access to both the Function and Depth Of Field Preview controls (Fn and Pv as Juerg notes) - the first time I have had access to these on any housing I have used. I know Fred B commented recently that he would like to see these controls on housings.
So far I have used depth of field preview for checking focus and bokeh on macro shots, without removing my eye from the viewfinder. I think it will also be useful for checking corner sharpness at various apertures when shooting wide rectilinear lenses. Not got my 16-35mm with me - so haven't tried that.
My favourite use for the function button, so far, is as flash off.
Actually both these buttons are programable on the D4, so I have swapped them over. This means on the housing I have to push the lever forward for depth of field preview and then pull it back for flash off.
The lever is just above the shutter release, so your spare index finger (I always shoot the Subal's with my middle finger) falls directly onto this lever. When you want to shoot a silhouette, you just pull back on your index finger as you release the shutter with your middle finger.
Hard to explain without a photo. Here is a photo showing my hand on the housing (low quality as I just took it hand held indoors):
It is very nice and obviously can be used in a fraction of a second (while still looking through the viewfinder and without taking your hand off the shutter) to get a silhouette and then get a flash shot instantly afterwards. Going to be great with mantas/turtles/divers etc.
Much more to write about this housing and using the D4 underwater, which just went through its 36,000 photo already! Will try to add some thoughts in the coming days.
Alex