Summicron sums it up well, but with a name like that he may be biased The Sonnars are truly excellent, better than anything Leitz had until the Summicron and Summilux. Having said that, my M2 is a much better user but I still regularly use the Contax (I have a 11A and 111A) to punish myself ergonomically! I only have a F2 Sonnar and the uncoated pre-war 1.5 Sonnar and also an 85mm F2 Nikkor.
If you can find a 11A instead they look prettier and the meter on the 111A rarely works anyway. I have two shots in the gallery that I posted a couple of days ago, one is Holly, with the 1.5 wide open and the other of the lone tree is with the F2 (coated) Sonnar.