[MatthewS]

Working on real projects, seems to be the only way to get to know it. Tutorials and guides won't help you integrate your needs to omnigraffle's features.

I learned it by informing my clients that visio couldn't the handle amount of fpo images needed (which was true) and that I was switching over to Omnigraffle. Using Pro allowed me to import existing Visio files and see how Omni shows the same objects.

Things like:
Visio's Page = Omni's Canvas
Visio's Document Stencil = Omni's Master Object
Visio's Background Page = Omni's Shared Layer.

Often, when struggling to reconcile what I already know in visio into how omnigraffle does things, I make a list of items I needed to do in omnigraffle.

On the whole it took about 4 weeks of 40 hours to get to a place where I felt confident to use, and able to learn from forums for more.