I have the entire Office suite for Mac, and it's WAY nice, I love it actually. Nice thing about using the Office suite is that it's fairly transferrable cross-platform so if you have a Mac at home and PC's at school/work you're all set. For me, all my clients work on Windows and send me content in Word or Excel all the time, so it's a necessity. If you're looking for free though, then maybe not, but I think I only paid around $150 for the "Student/Teacher" version - all four apps. I bought it from an Apple retail store that knew FULL WELL I wasn't a student or a teacher, and they just said I didn't need to register it.
I use it plenty, and it just blows PP away. But there's a caveat. 99.91% of the world uses PP, and while KeyNote can export to a PP format, you can end up losing quite a bit in the process. If I know that my final product will end up having to be shown via PP (especially if it's PP on Windoze), I'll just make it in PP from scratch.