OK so I am late to this party -- If like me, you use Visio's shapes and shape library to generate simple building plans to give to an architect to draw-up in CAD; of for schematics that can be built in minutes not hours - then OmniGraffle Pro is not an effective replacement for Visio.
For those who wonder what Visio is for -- Visio for me has been a stalwart companion since it was first released by Shapeware in 1992. I use it for real estate (conceptual site & building plans), financial and project advisory work and as a tool to communicate with draftsmen, architects and engineers -- with Visio I do not need autocad or equivalent, my professionals have this and use it well; but I needed a tool that helped me get my ideas neatly down on paper. My colleagues use Visio extensively for process mapping, org charts and other consulting uses - they also agree that there is no substitute for Visio.
As it stands today (15 May 2014) there is no product available on the mac that is equivalent to Visio for my type of use - I have tried and rejected 15 od called Visio replacement packages. None, including OmniGraffle Pro are as easy to use, as intuitive or as capable as visio. BUT, I am told not all is lost - you can run Visio on Parallels, together with the rest of MS's apps that MS will not make Mac compatible - this is what I will be doing shortly. My challenge to the app developers (including MS) is to strive harder. Given the penetration of MS Office for Mac, I am bemused why MS cannot be bothered to release a visio version for mac.... endless rumours, but no delivery.