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.