A few years ago, I was tasked with creating motion graphics for our digital signage (http://www.fourwindsinteractive.com/) and I had two options available: Photoshop or Powerpoint. Photoshop couldn't easily create motion graphics, not could I easily create transitions between displays. Powerpoint, on the other hand, could create templates and quickly input the text, but it all looked boring and drab.
And then I realized I had Keynote on my Macbook. It was not something I had used and I had expected it was like a simpler version of Powerpoint, but the few presentations I had seen before were crisp and clean. I ended up realizing that I could create presentations, export them as static images for the display, or even as a series of movies that could be used within my signage. Not only for transitions, but adding dramatic elements to the signage that was often subtle, but kept attention to it. Compared to Powerpoint, it was elegant...Powerpoint felt like a brutal hammer that got the job done, but was created by people with absolutely no artistic value. There are other tools out there that can do this, but I am not an artist. Nor am I in this world. I prefer to use the quickest application available that gives me the cleanest results. And this was it.
Since then, I have transitioned to ONLY using Keynote for any presentation, even if I have to eventually export it to Powerpoint format. It will lose a little in the translation, but still quicker to get the same results than using Microsoft's tools.
The web collaboration is the only way to collaborate with someone on another operating system. It is still in beta years later. It works, but barely.
Some of the more advanced features are hidden behind panes and popups and this makes it difficult at times to get great results.
Digital signage, presentations, and recently Keynote Live where folks can watch the presentation from any computer without other specialized software.