Personally, I prefer Omnigraffle over Visio, as Visio seems to actively fight making documents look nice, whereas Omnigraffle is much more aware of the grid and layout issues, plus the defaults are nicer, and it's easier to change things from the default. (Mac version, not the iPad version, which I haven't tried yet.) Using Visio is a chore, whereas Omnigraffle is more fun to use, i.e., it has a better experience. And as someone working in the field of user experience, the experience is important to me.
OTOH, I admit that this is partially personal preference, and both tools will get the job done. Other than the experience and the interface, they're really not that different, at least for the types of things we'd generally use them for.
Yeah, I have it and love it for quick and simple mockups on the go. I wouldn’t start a large project with it, but for getting something down quickly on a plane or on vacation it is pretty nice. I haven't tested out any export paths yet.
Re Omni-gfsfdfdsafdfas :) when I saw the $50 price tag I just about spit. Personally I can't imagine ANY app worth that price. Even Apple doesn't charge more than $20 for their full featured apps. Also, and not to start a debate though I'm sure one will come, I have a very hard time believing in any tool that is only on a Mac. I love my iPhone and iPad for consuming content, but for design I am a PC. I love being on the platform I am designing for in the end. So, why is the most popular UX tool only on the Mac? Adobe understands this. It makes no sense. It feels pretentious, so I won't use it. I feel especially strong about this given that the majority of companies needing high end UX, IA services are corporations and they are by far dominated by PCs. I wish it made sense from a workflow standpont.