Of course these are beautiful to look at classic old boats. I have never sailed on a Kettenberg and have only encountered them in written descriptions and seeing a few stray examples over the year. As much as I love wooden boats, and enjoy sailing truly traditional designs, the Kettenbergs represent a type of 1940's through 60's era racer cruiser genre that I really do not particularly like, except perhaps to look at.
While the Kettenbergs may be the exception, and that is unlikely, based on my experience with similar designs these short waterline, fin keel-attached rudder boats were a bear to sail. They tended towards heavy weather helm in a breeze, they needed to be sailed on their ear in order to get even mediocre speed out of them, they don't track worth a darn, they were awful to sail in heavy air, and very poor sailers in light air. If the boat has not been modernized, the sail handling hardware of the day was often dangerously undersized for the loads and modern safety systems were virtually absent.
These are the kinds of boats that you buy if you have a lot of money to throw at owning a boat, live in an area with predominantly moderate conditions, you are not interested in racing or cruising other than perhaps roughing it on an occasional overnight or weekend, and you can afford to take you boat out of commission for prolonged periods during the year and enjoy working on boats as much as you enjoy sailing them.