Hi,
As long as I've been looking around, BibDesk remains my preferred citation/reference manager. Unfortunately, I don't see an easy way to integrate it with Nisus.
What I've been doing is creating a dedicated folder in BibDesk for each essay I'm writing, and have all the needed bibliography references there. All references will also be available in the main list, so there is no problem in using the same reference in different writings.
Each time I need a citation, I drag/paste it in Nisus. In BibDesk I choose not to export the cite-key, but the final citation formatted with my own template. At the end, I will copy my bibliography list from BibDesk to Nisus, with my template applied. Apparently, this works fine. But I fear theer isn't the same flexibility of using cite-keys.
BibDesk allows the use of scripts for integration with any other app. There is currently no script for Nisus, while there are for Pages and Word. I tried to understand how to edit one of the exising scripts to work with Nisus, but with no success. Maybe someones else could be capable of doing it?
BibDesk is free (unless you want to donate), and in my view very elegant and effective. It just lacks this additional touch of flexiblity.
Paolo