Keynote is just so damn superior and more fluid, while PowerPoint does have some more advanced timing options and the emphasis animations nothing compares to Keynote 3, I've made quite some presentations in Keynote 2 but I had to resort to PowerPoint for technical reasons (for example, the computer I needed to display it in is a Windows machine) but now that I have a MacBook Pro I can just take that for presentations and use MBP as a presenter display and whatever they have in there as the main display and now that I started working with Keynote 3 and its new features, I'm amazed, work is fun now 
In my opinion, and if you are a heavy powerpoint user and do tons of presentations, Keynote is reason enough to switch to Mac, sure it still needs some work (more advanced options and better timeline options) but as it is Keynote>>>Powerpoint, as always Apple delivers with some incredible software and actually makes you glad you have to do a presentation with such software to power it.
In my opinion, and if you are a heavy powerpoint user and do tons of presentations, Keynote is reason enough to switch to Mac, sure it still needs some work (more advanced options and better timeline options) but as it is Keynote>>>Powerpoint, as always Apple delivers with some incredible software and actually makes you glad you have to do a presentation with such software to power it.