eleskin
Well-known
It seems from what I am reading that this Zeiss 50mm f2.0 Lens cannot be beat for the price, and is better than the 50mm Summicron, especially the used Summicrons made in the 1980's and 1990's. For what they go for used, I can get the Zeiss lens new. I am also looking at the Voigtlander 50mm Nokton, but I want sharpness at all f stops, especially wide open.
Patman
Established
I can't say I'm happy with the Zeiss lenses, today I noticed the whole front of a fairly new lens was wobbly when I took it off the camera it was loose and I had to tighten it to get rid of the wobble. I have 50 year old Leitz lenses and have never had any such problem, poor workmanship if you ask me.
oldoc
oldoc
No looseness on mine..sweet images though.
jplomley
Established
Another vote for the Planar. Phenomenal lens in every respect. After a year of solid use, no wobble to report here.
sienarot
Well-known
Or you can buy a used-but-like-new Planar (Shameless plug) 
payasam
a.k.a. Mukul Dube
While a new Planar will be pretty much a known quantity, a used Summicron could be something of a gamble.