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On the basis of all of these comments I used Gliffy the other night, and I must say it's really good. I am saying that as a fairly light user of Visio.
 
OminiGraffle Pro is about as good as it gets for Mac 'Visio'. It's nowhere near as good as Visio however (that's my opinion anyway having used them both). If you really want Visio then the best way to do this is to run it inside a Windows VM in either VMware Fusion or Parallels Desktop.
 
OminiGraffle Pro is about as good as it gets for Mac 'Visio'. It's nowhere near as good as Visio however (that's my opinion anyway having used them both). If you really want Visio then the best way to do this is to run it inside a Windows VM in either VMware Fusion or Parallels Desktop.

I use both, but you are correct in that they are not the same. Especially if you are sharing Visio documents with other Visio users. File compatibility is key here. Otherwise, I do like OmniGraffle Pro (I use ver 5) overall if you are not needing full Visio file compatibility.
 
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