Also, using iWork apps to read Office docs isn't really the same as opening them. When you drag an Office doc onto Pages or Keynote, it gets imported into a new untitled/unsaved document.
If you just want to be able to open files other people are sending you, it's no big deal, but you should keep it in mind if you plan on sharing and co-editing files with people. Because to edit a Word doc in Pages, you open it and make your changes, then use File->Export (instead of Save that you're used to), then either choose a new name and location for your file, or save it over the old one and deal with the confirmation dialog...