They aren't that special, to be honest.
I live near Cologne, about 20 min away. I's terribly full, especially full of tourists, and it's hard to walk. Plus the one thing that people from Cologne are known for is being unfriendly. And that is true, they are so rude.
The older parts of the city are are quite pretty, but they have started to take down all the old buildings to put up new and fashionable ones.