There's a lot of feature overlap between OmniGraffle and Visio, though I'm sure each has features that the other doesn't.
I know that some of our customers use the application for space-planning, garden layout, etc, though I don't know of any that specifically use it for the purposes you're describing.
OmniGraffle 5 Pro can open Visio documents, so you may want to start by just opening some of your existing documents in our application - it'll run in trial mode for two weeks - and see how they look.
Additionally, you may want to see if any of the stencils posted to Graffletopia have shapes that you could use. This section of the website seems most directly applicable at first glance:
http://www.graffletopia.com/categories/architecture