Omnigraffle is great for documentation, mapping, flowcharting, and other technical diagramming scenarios. It's simple enough to bang out a quick illustration and powerful enough to build complex blueprints for complicated technical systems. If you need cross-platform compatibility, though, you're probably better off looking elsewhere. If you want complex integration with data sources (ala Visio's SQL Server integration for shape metadata), OmniGraffle also falls short — but those scenarios are few and far between in my experience.
Proto.io is the perfect solution for UX/UI designers that need to create and share interactive prototypes. Proto excels at the interactive part. There are so many different ways to make elements change or interact, whether you're viewing the prototype on a computer (mouse clicks) or on a phone (screen touches). We could build and prototype our entire mobile app on here if we wanted to because Proto.io has the customization that we need to make the app look and feel like ours. If you're looking for a software to create wireframes and static mockups, Proto.io is probably overkill for you.
OmniGraffle is fairly simple to use, but the one thing I think it does best is working with curved lines, particularly if you are using some of the available arc templates. Drop an arc onto your page, then tell it the dimensions it needs to be, and viola! Done. Manipulating the arcs is as simple as clicking and dragging offset points.
OmniGraffle has also done an excellent job in stirring up the creative minds of many people who create templates and tools to work with OmniGraffle (not that Microsoft hasn't done so either), and managed to get the bulk of those into well organized repositories.
What it all boils down to for me is: it just works. One doesn't need to have a computer science degree to work with it either. It is as simple or complex as you want it to be.
There could be more scaffolding to support new users in getting to know how to best use all of the many features and tools it provides.
There could be more support for printing. Many of our process flows extend beyond just one page, and OmniGraffle is rather finicky about printing multipage documents.
We could use this tool more effectively if there was the ability to have real-time team collaboration. Document handoffs can be a point of missed communication, so it’d be great if we could ever have the option of working together in the same place at the same time.
use Omnigraffle for diagraming or for quick wireframe drafting
use Sketch for more in-depth wireframing that you can then flesh out into a high fidelity mock-up, export assets for production, and integrate with prototyping tools like Proto.io and UXPin.
Proto.io is definitely the premier mockup and wireframe application. Lucidchart shares some of the capability but can be a bit clunky. Proto.io is interactive and smooth and helps you to see exactly how your UI will look and behave before the development stage. Additionally, it's library of native components is robust.
