Build something that works
Markus, here are two helpful links you should check out:
1. StackOverflow link that explains one way to do it: http://stackoverflow.com/questions/16387058/rails-devise-user-profile-url-with-username-on
2. Rails Guides article on the many routing options: http://guides.rubyonrails.org/routing.html
If you implement this, make sure that you check that the usernames that people register with (and change to) are unique! Let me know if you have questions about this.
thanks, i'll update you with my progress