As for word processing
I was like you, deciding between MS Words or iWork Pages. I write my research papers and have it corrected by my professor in Pages. I basically just exported it into .doc file, no problems, all the track changes are there.
The only inconsistent thing I found (when exporting from Pages to .doc, then importing it back from .doc into Pages), and I mean it's the only ONE thing I've encountered, is when you insert pictures, in the Inspector, do not select any alligning option such as "picture floats above text" or "picture causes wrap around text" etc. If you untick those things, everything else is fine, including using Styles.
I found the user interface in Pages is nicer to use, Word is clumpsy and a poor attempt to try to copy the Inspector from Pages. I look forward to the Pages in iWork '09 have even better compatibility.
As for presentations, it's a no brainer, Keynote is by far much more superior than Powerpoint. I've never looked back even once!!
As for spreadsheet, iWork '09 finally features best fit trend line and error bars, so making Numbers more useable now. It's suffice for me. For more serious scientific graph plotting, you should use
Plot which is a free open source program and produces very professional looking graphs with all the options you want, rather than using Excel. Incidentally, the charts from Numbers do look beautiful, if you just wanna demonstrate something nice and simple.
So the verdict, iWork '09 over Office '08, keeping it real