I use Aperture in a very odd way. I use it to process my Canon RAW files before I convert them to JPEG and store them in iPhoto.
I then delete the originals.
Problem is RAW files cannot be used to archive images, and I really care about longevity. I'd prefer a better file format, but for I've been thinking JPEG is the best we have*.
In addition, when I bought Aperture I thought I'd migrate from iPhoto. Alas, Aperture does not support metadata associated with collections of images (albums) -- and that's important for us. Importing of iPhoto Libraries into Aperture discards important information. So I've stayed, grumpily, with iPhoto. (My Apple love died some years ago - no surprise to readers of this blog.)
Using Aperture as a kind of darkroom means I've never done much with Aperture keywords, but on a recent solo non-work related plane flight I took my Aperture Library along to catch up on image processing. I had a rare couple of hours to look at Aperture's keywords.
The first thing I noticed is that I could organize them in a tree (hieararchical, outline) structure using the (shift-H) Keyword HUD. It's a strict tree-type directed graph -- no mulitple inheritance. Any term (node) can be a keyword (the user interface and documentation are misleading on this).
Wonderful -- I thought. I'd prefer an acyclic directed graph with multiple inheritance, but a tree is nice. I can create an ontology of family relationships and execute inheritance queries -- so a query on "FL" will find all children of "FL" such as Emily, Ben, Brinna, Tim, Kateva, me, etc.
The tree is merely a way to organize terms. As far as Aperture is concerned the tree nodes (terms) are flat keywords assigned to images. All queries are term queries -- no inheritance, no subsumption.
Oh well, it does make it easier to continue my disgruntled data locked iPhoto relationship.
* I would love it if Microsoft's HD Photo (superb LOC description) were to be blessed by the Library of Congress. The LOC currently favors JPEG 2000 for lossy compressed archival images (NDNP profile specifically), but their next best choise is JPEG - which is what I use. I've another post due on this topic. After reading the LOC recommenations I need to reconsider whether JPEG is still my only effective archival option. (Update: Aperture doesn't export JPEG 2000! Weird, since iPhoto can manage JPEG 2000 and so can Quicktime. Looks like I'm staying with JPEG for now ...)