If you're actively collaborating with Word users on the Mac or the PC, Pages should play nicely most of the time; however, you may see some formatting quirks as you save back and forth to the .doc format. Pages does support tracked changes, which are crucial for most multiparty Word workflows, but if your document roundabout depends on them heavily, you may want to verify that your colleagues are seeing things the way they are supposed to.
While word processing is, well, word processing, differentiation for presentations often comes in the way the content is delivered, which can overshadow the substance of the content itself. For presentations, Keynote is, for most home office/small business users, superior to PowerPoint. It bests PowerPoint in what matters most in presentations: impact and style. Alignment guides help you best position objects, and intuitive animation and build schemes make adding some shine to your presentation that much easier. Like many Apple products, Keynote strikes a nice balance between usability and advanced functionality.
On the flip side, expert PowerPoint users can point to specific drawbacks in Keynote for professional-level production work: no way to gracefully play audio across multiple slides, no left-side or top pasteboard area to stage animations and no support for the Flash plug-ins and other third-party tools common on the Windows version of PPT. The return of Visual Basic macros to PowerPoint 2011 on the Mac means that some automation and add-on options will be present there (or easier to implement), versus the limited AppleScript support in Keynote. PPT 2011 has also added or improved features to match up with Keynote, including a version of the much-beloved Instant Alpha masking tool to knock out image backgrounds on the fly, on-demand web playback of presentations to remote audiences and more.
[One of the biggest feature improvements in PowerPoint 2011 isn't even in the app itself; it's in PowerPoint 2010 for Windows, which finally supports embedded QuickTime movie playback. You're no longer dependent on a tiny list of mutually compatible media formats or forced into transcoding your videos into WMV to collaborate with Windows PPT users. It's a major deal. –Ed.]
One major hiccup for enterprise or collaborative use: the compatibility story between Keynote and PPT, while adequate for casual use, is not so spectacularly good as to be relied upon for daily use. Keynote's animations often don't save gracefully out to PowerPoint, and there are frequently formatting and style mismatches when going between the two programs. If you have cause to round-trip presentations with PowerPoint users inside or outside your workgroup, you may prefer to work in the most compatible app you can find. If you're controlling your presentation from creative concept to final delivery and don't need to play nicely with others... well, then, Keynote is the way to go.
Most advanced PowerPoint users should have an easy time getting acclimated to Keynote, and may find themselves becoming vocal converts to the Keynote cause. The same can't be said about the Excel-to-Numbers transition. More advanced Excel users will likely find Numbers frustrating to use and limited in scope. For instance, advanced features like macros and pivot tables aren't available, and exporting from Numbers to Excel will sometimes result in a hodgepodge of different sheets in Excel.
In a Mac-only world, one where we don't need to exchange documents and collaborate with others, we could all just eschew MS Office and settle on iWork (except for the heavy spreadsheet users). But alas, the world isn't perfect, and we must deal with it.
The biggest annoyance for collaboration when using the iWork suite of apps is that while they can all read and export MS Office files (.doc, .xls and .ppt), they also have their own native file formats. When saving a file, you'll have to specify whether or not you want to save another version for it for use on Microsoft Office. Sometimes, recipients of your converted files may have some issues. Word-related issues may include font problems and formatting of objects (i.e., objects with shadows, reflections, etc.) and headers, while a lot of times charts don't come out the same when PowerPoint users try to work on a Keynote-exported version.
Standing on its own, iWork matches up well with Office: Pages stands up to Word's functionality, and Keynote runs circles around PowerPoint. However, Numbers is still no match for Excel except for very basic usage scenarios.
These things, however, must be weighed against the realities of collaboration, and one's purchasing decision should reflect this. iWork sells for $79, while Microsoft sells the Home and Student version of Office 2011 for a retail price $149 / $99 for the Academic version. As noted up top, Amazon has a great deal right now on the single-user Home and Student license. If you need the Business or Enterprise versions that include Outlook, those start around $185 -- but be sure to check with your IT department to see if you're eligible for a reduced-cost license before you shell out for the full-priced box.