I think the object diagrams are great. I would like to use them myself for future developments. You mention you use OmniGraffle to draw them, is there a specific template or have you created one yourselves?
I can’t quite remember where we got ours from, but I imagine someone has posted some UML stencils out there for Omnigraffle somewhere.
The three big things to have are the instance templates, the arrow with a label on it for instance variables and a dotted arrow with a label on it for messages.