An underdog, often overlooked because of its big brother the 85mm f/1.4, that indeed performs marvellously.
But this one is however one of the best Contax lenses ever made.
100% crops. Left center, right borders (on Canon 5D Mark II):
The Sonnar is compact like a 50mm lens, razor-sharp, has no troubles with flare, it is contrasty and has the marvellous Zeiss colors.
Its only problem is the maximum aperture, quite limited.
If this is not a concern for you then get one, and you will love it.