If you are registered, you get access to the members only section, can participate in the buy & sell second hand forum and last but not least you can reserve your preferred username before someone else takes it.
I have been using the Leica 21-35 for some months now and find it a really good lens for this range. As always, the Leica is designed for super performance to the corners and wide open apertures. An alternative is the CZ 21 2.8 which beats all the wides and a CX 35 1.4. If you don't mind changing lenses these primes will give at least as good performance as the Leica 21-35.
While I cannot comment on the C/Y 21/2.8 ( I haven't owned one for about 8 years ), I can relate experiences with the Leica 21-35 R Zoom per the question asked by Steve...
I use this lens on a Leica R9 with DMR digital back. The DMR creates a 1.37X crop factor which limits this lens to a a 28 to 48 mm equivalent. This effects the perception of both the distortion and vignetting because the part of the photo usually most effected by these issues is cropped off ... but not all of it when shooting at the 21mm end.
My perception based on actual application is that this lens is one of the best W/A zooms I've used to date ... outperforming any of my Canon wide angle primes, and any Zeiss W/A zoom including my previous 17-35/2.8 N.
It has the least barrel distortion I've seen on a lens this wide... better corrected in his regard at 21mm than my Canon 35L. Chromatic Aberrations are at a minimum including areas toward the edges when bright back-light butts up against a silhouetted hard edge.
Very crisp, and exhibits resolving power to the most minute detail right out to the edges (again the edge is a cropped in one in these cases due to the DMR sensor size). On the DMR it provides almost the exact focal length needed for my wedding work ... where a 21 mm creates too much "edge pull" to shoot people with (28mm being just about the threshold for such shots).
I often use this lens to show all inclusive shots of a wedding ceremony in progress ... and this is where Barrel Distortion used to drive me nuts, until securing this lens.
If I was limited to two lenses, it would be this one and the CZ 35-70 f3.4. And just to clarify, the upper left corner crop above is from a full-frame shot (EOS 1Ds II). It doesn't get much more demanding for a 35mm lens.
Many of my Arizona gallery shots, such as this one, were with this lens ...