[Zig]

As I'm writing on Day One of arrival, please bear in mind the largely subjective nature of my comments, though I've tried to document some of the testable bits as fairly as I can. I'll try not to "talk it up", as this might well be the tendency in we who have acquired something that is supposed to be nice because it's expensive...and it's to be hoped that readers will not be of the "what lens should I buy?" mindset: my findings would be of no assistance at all, I'm afraid!
I've "tested" this lens on a 350D body, so bear in mind that its full-frame performance will be guesswork only: if you see softening at edge/corner now, then expect more of it at FF if you're a peeper.
I shot all pics in raw, converting to jpegs in Canon's own software: everything standard, with white balance at Daylight and sRGB colourspace. At initial export to jpeg I used Canon's auto-Sharpness set to Low; no further pp or sharpening was added, except where I will have said so. None of the 100% crops received any sharpening. Conditions were lousy: dark, rainy, with very low contrast; all pics were shot using a single focus point and did not meter manually...so expect exposure fluctuations. I used a heliopan polariser for no other reason than protection(totally unhelpful, aren't I!)...but don't worry, guys: there was no light to polarise and I had it rolled off anyway. I shot at ASA 200; no adjustments for noise. The tripod, should you wish to know, is my Manfrotto with an 029 3-way pan/tilt/swivel head.
Alllrightythen:
Build quality= reassuringly chunky; nice firm focus and aperture rings; there was much less vignetting with the polariser than I'd thought...this took about a third of a stop of light from the corners(bear in mind that this is a 1.6 crop camera).
I've not experienced USM before: neat! I can see that street candids would benefit from this.
I can tell you now that the f2.8 aperture really helps viewing: makes life easier already; I can also tell you that at f2.8, even on a 1.6 crop camera, the upper corners are visibly considerably softer than the bottom corners.
Now, I hadn't thought this would be a big deal, and it isn't, yet this lens is the most distortion-free wide-angle of any type I've experienced. Nay bother, I hear you cry, as we've all got corrective software...BUT...it's sure nice to know that you can relax into the visible composition rather than imagining compensating for it while you shoot. And when I say "most" I'm including my experience of a Nikkor 24mm, my Canon 10-22mm zoom and my Pentax SMC Takumar 45mm medium-format prime lens. Of course, readers must now dial-in(guess) the fact that I'm still on a 1.6 crop camera.
The first 3 pics show what its angles look like with the Canon 350D: just to keep you interested, I shot these at f11.


...that was at 16mm.


...and at 25mm


..and 35mm

The above images were sharpened..but it needed only one pass.
I was interested and pleased to note that this lens has significantly less distortion than my 10-22(note comments on 1.6 crop as above)...it remarkably demonstrated hardly any chromatic fringing at either extreme.
Now...and I've thought long and hard about this...I am convinced of the following points:
1. this is the equal of my Canon 10-22mm in terms of image quality apart from their respective 2 widest apertures(well, I'll call it sharpness and resolution...but am not getting into semantics about acutance)....AND from around 20-26mm is visibly superior from f5.6 to f16.
2. At f8, at any focal length, it outstrips the Canon 10-22mm;
3. Its tele end at f2.8 holds up better than the wide end at both cente and edge;
4.My Tamron 90mm f2.8 macro at f5.6(its best aperture) is better centre and edge than my Canon 50mm f1.8 Mk2 prime at f8(its best aperture)..BUT...from around 23mm to 25mm the 16-35mm Mk2 is equal to it at f8, ever so slightly below it at f16.
5. At 25mm f11 the 16-35 is not only significantly better centre and edges than my Tamron prime at f5.6, it is also possibly the sharpest lens of all I have owned, rivalled only by my Pentax 67s 90mm prime.
6. At 25mm f11 the Canon has the odd quality of being both mindbogglingly sharp and also able to "peer into the shadows": perhaps not as much as a Leica 19mm Elmarit in the centre(and not at the edges) but, really, in the same ballpark.