Different strokes for different folks. Visio is the one piece of software I miss from Windows. I hate OmniGraffle as its boolean operations are pathetic and it seems to think it knows where to put stuff instead of where you put it. Visio also has so many more shapes.
Using Visio is like stepping back to 1999. They could have at least updated the renderer, added better magnets and auto positioning... It's bizarre looking at a old-style renderer bolted onto a metroish UI in 2013. What are their designers thinking?
Visio was the best diagramming tool in 1999, it's still popular in the enterprise, biz types love it; omnigraffle is more for designers, engineers who use macs, different markets to be sure.