Get into the habit of using Windows rival iWork - it works well
Now iWork does not contain all the tools Office does. It has only two applications: Pages, a word-processing and page-layout application, and Keynote 2, the second iteration of Apple's successful presentation application.
Office has Word for word-processing, PowerPoint for presentations, Excel as its spreadsheet program and Entourage as the e-mail client, calendar, contact and project support application.
Apple's free iApps - Address Book, Mail and iCal - do a valiant job attempting to replace Entourage and they are almost there. iCal, the strongest of the three, will probably win me over shortly with its cool network (and internet) shared calendars. Excel, the spreadsheet application, seems to be the best at what it does, even though there are more than 20 inexpensive Mac spreadsheet apps available if you want an alternative.
So it boils down to Pages vs Word and Keynote vs PowerPoint. I'll address the simple one first.
Keynote wins hands down over PowerPoint. It is colourful, has professionally designed templates and slick transitions, and its tight integration with other iApps makes the presentations you create spectacular. When you throw in Keynote's intuitive ease of use, low price and ability to import and export PowerPoint presentations, it is a no-brainer.