I had the opportunity to try Apple's Keynote for a presentation where I might normally have used Powerpoint or even OpenOffice. Rather than do a lengthy review, why don't I just summarize why the results are better than I might get from Powerpoint? After all, that's the ultimate goal with presentation software -- to do something Powerpointish, but faster and/or higher quality.
Let's leave aside interface polish in the UI (the Inspector windows) and the output (slick animated transitions). I don't think those are fundamental to the results.
IMHO, the strong points for Keynote are:
The outline view with slides down the left side in filmstrip style lets you nest slides into an expanding/collapsing tree. You can see the outline showing only the major sections of a long presentation, or see all the slides, or just expand certain sections. I immediately noticed when I exported in PPT format and viewed in Powerpoint, that the equivalent view there was incredibly cluttered, with all the slides plus all the bullet points from the slides visible in a single long list. The expanding/collapsing tree let me organize the slides more easily than with Powerpoint. Nesting related slides under a "key frame" (a slide with just a giant-sized title) helped propel the actual presentation, i.e. establishing that some slides were just going to come up for a second or two, with no exposition.
The font controls lead you very gently into the habit of writing concise text, and elaborating only as needed. Let me explain what I mean. With Powerpoint, it's a race to see how many bullet points you can cram onto each slide, by resizing the text box to allow more text. The generous spacing on Keynote slides sends the subliminal message that you're only going to get a few bullets on each slide, so make the text punchier. Then, if you're desperate to fit 1-2 more lines, you can adjust the paragraph and line spacing in very small increments until the extra lines fit. The page still doesn't look cluttered, and you don't have to endlessly shrink the font into unreadability.
The export options let you choose output formats like PPT, Quicktime movie, PDF file, individual images, even Flash. Although I didn't use these, I can see that they would be tremendously useful. I exported to PPT so my audience could view the slides with Powerpoint or OpenOffice. Depending on the context of the presentation, I might have chosen Quicktime movie or Flash, to post the presentation on the web for viewing directly in the browser.