I borrowed a Nokton from a friend for 6 months, simply put, it has never failed to deliver.
The news of the Sonnar came out just when I had to return the lens, being addicted to the speed, I jumped on one when it came out. It wasn't until I bought one did I know that the front focus is not just a faulty batch.
The Sonnar does draw beautifully, but I couldn't get over the front focusing. But I didn't buy a Nokton, perhaps while it's technically very good, the results don't seem to have the character the Sonnar has. I came across a few photos taken by the 50 Summilux IV and loved them, and finally, after four 50s, I'm truly happy.
Sonnar or Nokton? Depends on your preferences. One draws beautifully but front focuses, one is technically good but a bit big and doesn't have the same sort of character.
Here are a few photos by the Nokton, not sure if photos from Deviantart would show, I'm wary that three may take up too much room. I'll try one and put links for the other two.
http://fc04.deviantart.com/fs11/i/2006/206/8/f/2_2_by_kevinwong.jpg
http://fc06.deviantart.com/fs11/i/2006/237/c/2/3_2_by_kevinwong.jpg
And this one is taken by the Sonnar (excuse the size and the colour cast)...