Depends on whether or not your employer uses Windows…if you work for a large company or a company that uses Exchange, Outlook, etc., then it may be worth it to have MS Office 2008 Mac on your computer. Exchange seems to play nicely with Entourage (Outlook for Mac), and not-so-well with Apple Mail/Calendar (which is the default calendar/mail programs on Mac OS X, which requires IMAP). Going back and forth between your docs is relatively seamless with Office 2008 Mac. Another nice feature (albeit a small one) is with Word 2008 Mac, you can use “notebook” layout view, and record audio notes while you type; as you append bullet points and your outline, the audio notes are “chapterized” with the notes-awesome for meetings!
My experience has been (and I own both) that although Pages and Numbers can both open and save .doc and .xls (or in the case of the new versions .docx and .xla), in many cases, the docs don’t look exactly the same when you send them on to a Windows MS Office user. Although this may change with time and updates, as for now, it’s somewhat dicey proposition if you’re doing heavy file exchanging between Mac and PC users.
Like PnT mentions, iWork ‘08 is really quite impressive for the price, and Keynote ‘08 is the stuff-visually and functionally light years ahead of PowerPoint. If you’re only working on your own, iWork ‘08 is the way to go for individual work. Pages has a lot of cool templates for various kinds of docs and to me, seems a lot more intuitive and powerful for good looking design than Word. Lastly, you can very easily export documents in iWork to your website if you use iWeb.