I am curious to those that mention the 17-40 and 16-35 II are not sharp in the corners compared to the Zeiss 21mm, what f-stop were you using to compare to make the statement? Was it wide open or stopped down to around f/8-f/11? From what I've seen, the Zeiss only handles chromatic aberration and slightly less distortion than the Canon equivalents in the corner, but in regards of detail/sharpness they are pretty much similar stopped down to around f/8 (which I would assume most would use the lens for in landscapes - with the exceptions of astrophotography, low light events, or portraits).
In my personal usage, I have passed on owning the Zeiss 15mm and 21mm as they were not weather sealed like the Canon equivalents. Taking the lens out to destinations right up close to the ocean shoreline, dealing with mist from waterfalls, in rain, snow, etc. didn't seem practical in the long-term for usage.
For the price of a new Zeiss 15mm + $300+ filter, I decided on a used 16-35mm II + 17mm TS-e to cover my wide angle uses (16-35 II when I need filters & versatility of a zoom for more outdoor landscapes and the 17mm for urban/interiors).
By the way the 17mm TS-E can use filters with the compromise of slight vignetting at certain degrees of shift. This link can provide the DIY details:http://www.google.com/translate?hl=en&sl=de&tl=en&u=http%3A%2F%2Fwww.linsenschuss.de%2Findex.php%2Fblog%2F79-canon-ts-e-17mm-f4l-filterhalter