Forget function for the time being. This application is gorgeous. For example, check out the toolbar icons at right. The '09 versions (bottom) are less "busy." The thick border around the "Themes" icon is gone. The lines inside the "Masters" icon are thinner. Even the "View" icon uses a brighter blue (#93dbfc vs. #a0d2e9). Also note the addition of "Guides" in '09. Use it to turn guides on and off or enable different options.
It gets better. Select "Rehearse Slideshow" from the "Play" menu and you're presented with a display that blows the '08 version out of the water (you'll find comparison screen shots in the gallery above). From there, you can view the current and upcoming slide, presenter notes, the current time and the elapsed time (or time remaining if you've got a target limit). Go even further and customize this screen by selecting "Customize Presenter Display." I love it.
But how does it work? When you first launch Keynote, you're presented with the Theme Chooser, which has borrowed a few design elements from iPhoto. First, you can scrub over a theme's icon to see all of the templates in that theme, similar to scrubbing over an iPhoto event. A new slider lets you resize the theme icons, and the whole thing features that beautiful grey theme. Also noteworthy is that the Theme Chooser stands alone in Keynote '09. In '08, it hovered above the main editing window.
Actually building a slideshow isn't much different that it was in '08. The new transitions are nice, especially the text effects, but the mechanics of assembling a slide are very similar to last year's version.
The new iWork.com feature is pretty neat. Once a project is complete, you can click the "iWork" icon in the toolbar to have a version uploaded to iWork.com. You'll be prompted to send an email requesting someone else to review your work.
The web-based version of your project looks exactly like the real thing, and can receive notes and comments from any user invited to participate. I can see this becoming a great collaboration tool for remote teams, but I doubt I'll use it personally.
Now for my hesitations. Keynote has a lot of sweet eye candy, and the temptation to overdo it with nifty transitions is terrible. There's a real "because I can" mentality that takes over, like a cartoon devil on your shoulder. "Just one more bounce transition," he says. "Ooh, and that text sparkle. Did you try the water droplet yet? What about Cube? Duuuude."
I've seen Keynote presentations where the content was overshadowed by the flips and tricks that accompanied every new bit of information. That's really the fault of the presenter, though, not the software. I think.
My advice is this. If you're the type who builds lots of these things, you'll enjoy using Keynote. It's fun to put presentations together when the tools are this pleasant, and the integration with iPhoto and iTunes via the Media Browser is wonderfully convenient. Those who enjoyed Keynote '08 will absolutely love '09. You can download the 30 day free trial here, or buy the package for $79US (unfortunately, there's no special upgrade pricing). iWork '09 requires Mac OS X v10.4.11 or Mac OS X 10.5.6 or later.
Just go easy on the flashy bits, OK? I'm begging you.