If your PowerPoint presentation has lots of effects and builds then Keynote is the way to go. I just finished a corporate training video using Keynote to export the PPT and it worked great. As mentioned above though, some tweaking might be necessary since there are so many versions of PowerPoint floating around and not of the features or transitions in one version will necessarily translate to a different version, especially when going from PC to Mac.
iWorks is pretty cheap as well, about $80 retail. Well worth it.
Tom