Even if it were great, the fact I can't use it on my company workstation? Useless. Or even if I happened to have a mac workstation - being unable to share the source files with colleagues? It's dead on arrival. So basically it's a great diagramming tool so long as you work at a company that exclusively uses Macs. Which there really aren't many of. It's basically a diagramming tool for employees of Apple. Who, by the way, produce their own similar (less good) tools.
The productivity tools? Fine, you don't need to be able to share that. But Omnigraffle is juts a bizarre decision.
The type of person who is willing to pay extra for the polish Mac/iOS has over Windows/Android, is usually also the type of who is going to be willing to spend more for a better app.
But if it wasn’t a native Mac Cocoa app, I would never use it. A lot of OmniGraffle’s value comes from UI that abides by platform guidelines and the high-quality rendering and integrations enabled by being a native app rather than a lowest common denominator cross-platform port.
Omni in particular makes extremely clean, friendly applications that feel aligned with the platform.