Keynote is a more advanced presentation tool than PowerPoint. It integrates with MacOSX, and even allows you to control the presentation from an Apple remote or an iPhone. My favorite feature is that I can embed a playable video file into the presentation, which eliminates the need to have a web browser open in the background. I am able to show a video to my audience without having to stop my presentation and go to the web browser, then return to the presentation, and try to find where I was.
iCloud integration could be better. Presentations lose features and slow down significantly through iCloud. Though you are able to import and export powerpoint files, many features and fonts are lost.
If you are primarily a Mac user who has to present to colleagues, conferences, or students, Keynote provides an easy to use tool that is more powerful than PowerPoint.
As a classroom teacher, I use presentations on a daily basis, both to teach my classes and also to provide some sort of visual aid in the classroom. I also use Keynote when I present professional development activities for my coworkers.