I used to have MS Office on my Mac. I ended up dumping it for AppleWorks. I can't justify 300+MBs for something I don't use all that often. I've actually found AppleWorks to be easier to transfer files between Mac -> PC or Mac -> UNIX. I also have OpenOffice.org installed, but the 600+MBs it takes up is driving me nuts. I'm going to move it to my iPod soon. I need OOo however for work (where we use StarOffice) so I can't be without it.
Honestly, I love AppleWorks, no suite comes close for the price or the HD space. I actually like the Presentation software in AW, however I used it to create slides for a presentation in English Class in December. I haven't tried Keynote, but if you do "online" presentations I'd get it, if you are just making slides, AW will do you fine.
MS Office is not worth the money ($200+), AW is free with your iBook and Keynote is only $99.
TEG