OK guys, I just spent the day shooting this lens in a testing sort of a way, and also comparing a lot of files from it to those shot at the same places from the Nikon 24-70 F2.8 on a D800E.
Many things to say:
At 100% the Sony files look a bit more processed, even with the same develop settings. I suspect more RAW pre-cooking is going on. But by 50% or, I am strongly guessing in print, that disappears.
The Sony lens has much more distortion so if you correct it, you'll lose frame width and slightly compromise the very good micro contrast. It also has a lot more vignetting at most lengths and apertures, and the correction of that will boost noise more in those areas though at low ISO and 50% it's hard to see.
Then I did the most peepy thing I've ever done. I compared my harbourside series between the two and though the series were shot from slightly different positions on different days, they were all 'best of a lot' and were all tripod, ISO 200. I looked at them with no lens corrections and the same sharpening and Camera Standard for both.
I looked at 24, 35, 50 and 70mm and at F8, F5.6 and F4.
I gave a simple S>N or = or N>S for each pair.
On centre, out of 12 comparisons, there were 9 S>N and 3 =
At the edges there were 5 S>N and 4 N>S and 3 =
Nikon edges tended to be better than Sony at 50 and 35mm
Sony edges tended to be better than Nikon at 70mm and 24mm, by a big factor at 24mm
Neither lens is wonderful but as I work towards my holy grail of 'what's the best mid-range zoom on a 36mp sensor' I am homing in on the Sony, despite its many irritations.
The bad news is that I casually shot a hunt that was passing through today. The light was very poor, and I was running and gunning with hopeful shutter speeds and OSS and middle ISO, but very few frames were good. I think the F4 means that trying to AF on moving subjects in a hurry in poor light isn't a great idea but in good light, the lens is a reliable focusser and is always very sharp on centre at any reasonable aperture and every focal length. The edges are a bit disappointing at the extremes of the range but seem a touch better than the Nikon, and in the 35-50mm range where the Nikon has slightly better edges, the Sony's are easily good enough.
I have a feeling that this is going to be a complex lens to master. I have left side weakness that comes and goes and might be down to the angle at which I hold the camera but I think it's more likely some optical issue. But you really have to look for it and it isn't by any means always there.
My next task will be to shoot a comparison at F5.6 and F8 and 50mm versus the 55 F1.8 and though it certainly won't be as good, I suspect it'll be closer than people would guess...