i've been waiting for months for B&H to get stock and finally ordered elsewhere at $600 -- and shortly after i received the lens, [email protected]
has stock at $550!
actually, i've tested 2 copies which have very little between them (on 5D2) and are consistent with the Photozone test and Lloyd Chamber's report.
i was primarily looking for a lens to replace the 17-40 for travel as it's one too many bulky lenses in the bag that doesn't get used a lot
-the 17-40 as a bit better center resolution but both are good enough that this isn't an issue
-the 20 is better in the corners than the 17-40 at f4, but not as good at f8, but you've got to do some serious pixel peeping to see a difference
-both have significant CA in the corners
-both have mustache distortion although the 17-40 is more barrel with a little pincushion at the corners, while the 20 splits the barrel and pincushion and has a sharp distortion up-turn in the corners
as a reality check i compared my Sigma 12-24 and the Tokina 11-16 (on 7D)
-my Sigma is not a particulary good copy and was significantly worse in all respects
-the 11-16 and 7D was not as sharp (but sharp enough) in the center (as expected) but was comparable in the corners (the 17-40 and 20 had the same relative performance at 32mm effective on the 7D as the 5D, but most of the mustache is cropped out along with the worst of the CA)
Photozone and Lloyd Chambers agree that the Zeiss 18 is much better in the corners at larger apertures, but not significantly different stopped down to f8
the build quality and feel of the focusing ring of the 20 is outstanding, but the $50 lens hood can't be used with a lens cap (on the other hand the huge diameter of the 17-40 lens hood is a continual nuisance)
in general, the autofocus and versatility of the 17-40, at reasonable cost make it a clear choice if you don't mind the weight and bulk - although i must add that you should test the lens carefully (time consuming) - as is characteristic of wide angle zooms, manufacturing tolerances often result in seriously assymtercal performance which can vary with focal length. mine has been to Canon once under warranty and they improved some focal lengths and made some worse - and it will be going back again.
the Voigtlander 20 is no bargain even at the B&H price (on the other hand look at the cost in $, size, and weight to get better large aperture coner performance with the Zeiss 18)
if you can live with manual focus, the Voigtlander 20 and 40 paired with T2i seem a reasonable compact, light weight alternative to the lower performance 4/3 cameras
i'd like to see a reasonable sized wide angle prime with minimal distortion and excellent corner sharpness, but since i don't see this happening soon i'm going to give the Voigtlander 20 a try