One thing I like about this lens is it requires minimal post processing to deliver an image without distracting artifacts. You can't say that about most Canon UWA lenses.
One thing I don't like as much is the sunstars, at least from what I've seen thus far. The rays look thick and not as commanding as the sunstars of the EF 16-35mm f/2.8L II. Of course that lens requires some significant post work to remove CA, add sharpening and is not as sharp in the corners at wider apertures. The best one I've seen of the 16-35 f/4 IS thus far is here (16-35 f/4 IS on left, 16-35 f/2.8L II on right):
I have to say, there have been times that the 16-35 f/2.8L II has given me sunstars that were unexpectedly long and in ways intruded into the rest of the picture - but some of those shots actually turned out more interesting as a result of the impressive yet not purposeful sunstar.
So, Canon Rumors members... Who can impress me with some bombastic sunstars from this lens?
Ruined, the first post in this thread shows a sunstar comparison between this lens and the TS-E 17 f/4...but the links were broken until just now.
Also, I agree with the post processing - if you look at the last shot I posted, it would have been a CA nightmare with the 16-35 f/2.8 or 17-40, but with this lens, there was hardly any CA to remove. The big deal with that is that I exposed to the right to get the highlights, so the shadows had to be punched up quite a bit. With other lenses, I would get some nasty artifacts around the highlight edges from the removed
The vignetting and distortion are a bit higher than I'd like at 16mm, but DxO & ACR handle things quite well.
Overall, I'm very pleased with the lens, though I have yet to make much use of the IS.