Overall Satisfaction with OmniGraffle
Omnigraffle primarily used as a Visio replacement. It works well for flows and charts, as well as basic graphics. I have also used it for wireframing activities, but recently changed to using Axure RP or Sketch for wireframing and design activities as they have more flexibilities in those domains. Omnigraffle does offer much better visuals (as of 2014) than Visio.
- Diagrams flowcharts: you can do anything that Visio does, only quicker, easier, and better looking.
- Wireframing: while I prefer other tools now, Omnigraffle has a robust user community and offers free/paid widget sets for web, mobile, and desktop application wireframes.
- Basic interactions: Omnigraffle allows for masters and basic hotspot interactions. If you only need to show simple functionality, Omnigraffle can work.
- Interactions: each new visual must be on a completely new page (as of version 5). There isn't a good way to set up simple on-page interactions (i.e. show/hide of a widget).
- Defaults: while the default setting for the widgets are nice looking, they can look a little cartoony sometimes.
- Ease of Communication: makes it easy to show clean flows and diagrams to clients for increased understanding.
- Visually appealing: let's face it, clients respond better to visually appealing deliverables, and flow charts are no exception. Omnigraffle makes looking good easy.
- Axure, iRise and Sketch
Omnigraffle handles flows and diagrams better than other tools I've listed above. However, Axure is far superior for creating wireframes and turning them into interactive prototypes (basically "coding" using if/then logic statements). Sketch is much better for detailed design and pixel-perfect work, especially for web and mobile applications. I haven't used the latest iRise implementation, and while I prefer Axure, iRise allows easy integration of external data and spreadsheets for populating widgets.
If my colleague is planning to create work flows and / or diagrams, then I will definitely recommend Omnigraffle. It excels for those use cases. If they only need make a simple wireframe, I can recommend Omnigraffle as a decent dual tasker to supplement flow charts. However, if the need is to create more detailed wireframes or prototypes, there are other tools better suited in my opinion.