[gurtch]

On Erwin Puts web site he tests the ZM 21, and has good things to say about it. I quote Erwin :

Conclusion
This Biogon lens is second to none in the current rangefinder field and is a most interesting design in the 21mm class. It delivers at least the same overall performance as the Leica Elmarit-M 21mm, but without the employment of aspherical surfaces and at a lower price. The sturdiness of the mount may be not as good as that of the Leica, but the mechanical accuracy is not affected and I did not detect any decentring. With the Leica Md on the not-too-distant horizon (we may assume) it is a tempting alternative to the Elmarit 21mm, especially because of its even coverage over most of the image field at the wider apertures.

The performance of this lens warrants a somewhat wider perspective on lens design. Zeiss demonstrates that it is possible to improve on first-class lenses while using conventional designs and without elevating the cost to astronomical heights. Leica has lived for a long period in an ivory tower, supported by a loyal following of collectors who were more interested in the safety of their investments than in the quality of the images that could be produced with Leica equipment. The digital tsunami has at last reached the parapet of the Leica fortress and it is evident that Leica must become a more market-oriented company. Leica claims to offer an unique mixture of traditional values and modern technology for its products. You do not get market share in the current photographic world without a sensible price-performance relation and here Zeiss is beginning to lead the way, at least in matters optical.

Hope this helpful. I bought the ZM 21 for use with the M8, but I have not yet received the camera, so have not personally tried it.
Dave