For a late 50's lens it is very good.Not that sharp wide-open, edges tend to "fuzz" out a bit. Contrast is medium and the look of a negative is truly 60's.
Unfortunately it has taken on "collectors" pricing and though a decent lens. it is not worth that. If you are planning to use it for pictures (not a shelf decoration) go for the Summicron Version IV (post 1980) or the version III.
There are several better alternatives for the Summicron's. The Hexanon 35, the Voigtlander 35/1,4 or 2,5 and of course the best f2 around, the Zeiss Biogon 35mm f2.0. Also, the new Summarit 35mm f2.5 is supposed to be very good (i havent tried it yet).