I haven't used it, but when I was looking at potential wideangle primes, I looked at the Zeiss Distagon 18mm and 21mm. The 18mm was a little too wide for my purpose at the time, but it was quite a bit cheaper than the 21mm and judging by reviews, the image quality was almost as good. I'm also considering selling my 17-40, as I don't use it as much as I used to, since I got the 5d MkII, partly because filters are more problematic, but also, the landscape around here doesn't suit extreme wideangle in many places (although there are some places it works). Full frame dose also show up it's corner softness, so the 16-35 MkII might come into my plans at some point or I might reconsider the Zeiss 18mm.