There's a new review online of the new Zeiss Planar (50mm f/2) vs. the Cosina-Voigtlander Nokton 50mm f/1.5 and the older Voigtlander Color Skopar 50mm f/2.5:
http://dc.watch.impress.co.jp/cda/review/2004/12/27/634.html
Scroll down to the bottom where they compare photos from all three lenses at various f/stops using an Epson R-D1 digital camera. Key from the left to right: Planar, Nokton, Skopar
To my eye, the Planar is noticeably sharper, especially in the field (outer areas) than the Nokton. Note that the R-D1 has a tendency to reduce resolution in the field because its digital sensor does not handle off-axis light well.
