What do you intend to do with the software?
iWork Advantages:
Universal so it's full-speed on Intel Macs.
Apple-designed UI fits in better with the Mac.
Simple and lightweight.
Relatively inexpensive.
Office Advantages:
The standard office suite on Windows and Mac, so no compatibility problems
Very powerful with tons of features
Includes a spreadsheet app (Excel)
I personally use and like MS Office for the Mac. That said, if you'd like to save a little money and your needs are not too intense, go with iWork.
As for the suggestion to use NeoOffice, be sure to check it out too. I personally don't like it, but for some people it's fine. It's getting better with time (especially in the interface) too.