The University of Hull's institutional repository (IR) uses three forms of Fedora content object: simple, compound and complex. Our view is that the most complicated form that a content type can have should be the one that drives the decision. Whilst many Electronic Theses and Dissertations (ETDs) need only a simple content object, some require a complex one: thus all our ETDs are complex objects. Similarly, most of our past undergraduate examination papers need only be simple objects but some should be compound: thus all such exam papers are compound.
Simple objects (aka Blobs)
I just hate the term 'blob' but live with it because we've found nothing better! Blobs are simple Fedora objects, which is to say they have only one content bearing datastream for delivery. We use these for all manner of things:
Academic research articles
Dissertations (undergraduate, text only)
Guides and documentation
Policies and procedures
... to name but a few.
Currently we have three use cases for compound objects (single Fedora objects but with multiple datastreams holding content for delivery):
the original, plus three more at different resolutions
past undergraduate examination papers
the paper, but sometimes with a supplementary document (perhaps a case study) that must not be separated from it so that copyright agreements are honoured. Because some objects need to be compound and not just simple, all exam papers are treated as compound.
we have a very small number of library books that have been digitised because the irreplaceable original is literally falling apart due to use. Each page is scanned and turned into a PDF, the pdfs are then grouped, maybe into chapters. All the groupings for the book go in a single compound object. One could make an argument that they might better be a complex object, but that's the way we did it on the day!
At present the only mainstream example of complex objects is electronic dissertations and theses. The thesis PDF is one child and there may be other children containing supplementary materials of one kind or another - in practice usually video or sound materials.