The Retina 50mm F2 Xenon on the folding cameras is absolutely superb, and the 50/1.9 for the S-Series is even more superb. I find them sharper than the Xenars. The Xenar has a softer, low-contrast, pleasant look for portraits- but is not as sharp as the Zeiss Tessar or Leitz Elmar.
If you have an S-Mount camera, the 85/4 Tele-Arton and 35/2.8 Curtagon are also worth picking up. The 28/4 is soft, and 135/4 has a close-focus of 14ft.
5cm F2 Xenon on the Retina II
I'll create an album with the 50/1.9 Xenon. I also have shots with the 45/2.8 Xenar on the Auto II