I have used it for years and years, for normal stuff like software diagrams and business process and shit like that, but also for stuff like floor-planning my house and doing mockups to decide what furniture and TVs to buy, to making easy-to-understand political comic books to influence how my friends vote (old example: https://masonmark.com/stuff/2008/America.pdf), and I even used it to create the CV which helped land me the awesome job I have now (http://www.fivespeed.com/mason/resume/mason_resume.pdf).
According to rumor, the principal guy behind OmniGraffle had a fairly unusual employment arrangement and wasn't typically seen that often in the halls of Omni — I don't know the details, or even his name, but as an OmniGraffle-dependent user I'm wondering with some trepidation if he was part of these layoffs.
I happily pay for it.