I just bought one of these to give me wide options beyond my 24-70 L II and I'm very pleased with the performance. It has a surprising ace up its sleeve and I will comment on this a bit later on.

First with the ugly:

AF is frankly poor. Its nowhere near the quality one gets from Canon USM. Its noisier, slower and often inaccurate. However, I bought this lens for landscape use so I am much more concerned by the images it creates when focused through live view. The AF seems to be much more reliable up close than at distance. At 28mm and with a distant subject, you are just asking it to miss. If I got a bad unit regarding AF, I do not care one bit. The reason is down to its optical performance.

It is also heavy. And no easy solution for filtration. but you knew this already. At 900g or so, its far happier when the camera is tripod mounted.

Now with the good:

The lens is weakest at either extreme of the zoom range and I found my results comparable to what photozone.de found. At 16mm, you need to stop down to f5.5-8 to get a nice crisp image across the frame. Central sharpness is a little weak at wider apertures and corners are terrible wide open, but it does sharpen up nicely. At landscape apertures it is plenty sharp enough at 16mm, but falls a sliver short of what at its sweet spot.

Narrow your field by a mere 1mm and you get into the 17-21mm zone where this lens it at its best. It is razor sharp across the frame right into the very tips of the corners. Its at its best on centre by f4-4.5, but the corners peak a stop and a half or so further down. F5.6-F8 gives you everything you could ask for as a landscape shooter.

By 24mm the lens is just beyond its best but only a touch behind my 24-70 II (which is sharper than any 24mm prime I have ever used, with the exception of Leica M). By 'behind' I mean that at 100%, looking really hard at the screen, you can see the 24-70II is the sharper optic, but for all practical purposes it will make little to no difference.

At 28mm you do need to stop down as things are getting a bit weaker here. Still very decent, but the24-70II will be the better option here and you will see it on large (ish) prints.

The surprising bit:

At close distances, wide open and in the centre, the lens is tack sharp. This is the bit that matters when it comes to bizarre up close portraits, whether person, or object. F2.8 at infinity and focused with AF, is no use for anything (and the copy I have is poor in this realm), but up close (even with AF) oh boy, it is SHARP on centre! This means that the lens has (for me) weaknesses that don't matter (infinity, in the corners at wide apertures, esp at 16 and 28mm ends - but who cares about this?) and strengths that shine for when almost everyone would use this lens (wide open, close in, or stopped down a little further away)

Behind my 24-70 II L this is easily the sharpest zoom I have used on my Canon and I am pleased as punch that I did not get a 17-40L or 16-35 II. Performance at f5.6-8 from 17mm-22mm is, quite literally, perfect and neatly replicates what my 24-70 II does with longer focal lengths.

Photozone did say they had a number of dud units and were irritated by QC. Maybe now that lens production is settled down things are better, or maybe I was just lucky, but if you get a good one, this lens is clearly a stellar performer.

PS mine needed a +7 adjustment on microfocus.