Hi Peter, Marc, Jono, M (and hope I didn't leave anyone out)...
I think we can all agree as Marc said, that it's horses for courses, regarding lens choices and what our expectations and needs are regarding optical performances and parameters set by both the optical enginers that design these lenses as well as optical limitations in any one particular design. There are tradeoffs to be sure, but how big they are depends on our own personal use. Certain distortions found problematic for architecuture use, may be acceptable or hardley noticed in pictorial type imagery, or at the very least exploit its use for creative purposes. Then again it depends what kind of distortion we are talking about, how severe and whether it can be corrected, if necessary. So many factors go into choosing a lens thats right for a particular application.
Marc, I did mis-understand you, as in sending the Leica 18mm back for repair, as opposed for a refund. I can't say whether the Zeiss 18mm for SLR has the same characterisrics as the M version, but the 18mm Zm is a formatable optic and one where distortion is held down to respectable levels compared with its Leica counterpart. As for how it draws, it's farily typical in Zeiss fashion...a bit higher contrast than the Leica, with terrific resolution, basied slightly for the center of the frame.
I agree with Jono, and others that the WATE doesn't have quite the issue with severe distortion as the Leica 18mm does and as such, can be used in a wider set of applications, in my opinion. What distortion it does have, can often be delt with, when required, in post processing. I cannot say the same thing regarding the Leica 18mm.
As for the tradeoffs of the 21 and 24mm Lux's...vs. their f2.8 counterparts...I find them more than acceptable, for as Marc so aptly put it, they were designed for those that require their low light-shallow depth of field characteristics (among others). There are always tradoffs when we talk about ultra fast counterparts to normal speed lenses in a given focal length, especially in the wide angle arena...whether they be optical, size, handling etc. I'm reminded years ago about a optic designed by Pentax...their FA 85mm f1.4 lens. People wondered why between f1.4 and f4, shots taken at mid-infinity distances were very soft, but at close range, had both incredable sharpness and yet asoft diffuse glow, that made it one of the most incredable portrait lenses around...something the optical designer at Pentax stated he strived for when questioned (although Pentax never mentioned the purpose of its design) Desire a better all around 85mm, then their 85mm f1.8 was the ticket. Even the Nikon Af 85mm f1.4, which I used and compared with....couldn't hold a candle to the Penatx FA 85mm f1.4, when it came to portrait work (nor do I believe that optic is the best choice for portrait work). Point is, picking the right lens for its intended purpose and work with it's optical characteristics, exploit them to their best advantage, is how any lens should be used.
Obviously, Marc, Jono and many others here have done just that and their work illustrate the point better than any printed words can. It fact, many of their images not only illustrate, but inspire!
Dave (D&A)