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ardour.org, we recently switched from using Drupal forums to Discourse. The results have been delightful. We managed to get single sign on integration with our existing site working, which was important for us, but probably not so much for a GTK Discourse. More notably, once you figure out how to get the settings right, the integration with email was also a pleasure. The move has definitely made our forums more active, easier to manage, nicer to interact with and has also largely solved the spam problems that we had before, even when we used Mollum. So ... although our needs and goals were slightly different from yours, I can say that for us, the move was very successful and we like Discourse very much.
