I consider myself a power Keynote user, and overwhelmingly prefer the app over PowerPoint (on both the Mac and PC) -- even though though I'm just as well-versed at PowerPoint. Only on rare occasions do I start my presentation workflow in PowerPoint (if the deck is going to be chart-centric, to avoid the limited axis and error bar support in pre-09 versions of Keynote).
While both Keynote and Powerpoint get you from Point A to Point B, it's the "little things," such as alignment guides and better graphics support (i.e., native support for Photoshop PSDs and Illustrator AIs), that make the presentation journey that much more enjoyable and more presentable.
One of these little things is Presenter View. Although PowerPoint has a similar feature (in both Mac and PC versions) it lacks the polish and ease-of-use found in Keynote. Presenter View allows you to look at your slide's notes, and upcoming slides and builds, without your audience's knowledge.