Unfortunately, Apple's approach to interoperability somewhat undermines the usefulness of these apps for those of us who need to share documents with Microsoft Office users most of the time. Although you can send files to colleagues in Office's (now outdated) .doc, .xls, and .ppt formats, Apple's apps handle the process in a weirdly disjointed way that forces you to save separate copies of your work in the Microsoft formats and prevents you from simply creating and working in a Microsoft Office-compatible file. All of that preserves a sense that you're not really working in an Office-compatible app. It forces you to create multiple copies of the same document, and it increases your chances of distributing outdated copies of important documents you're working on.
As for file compatibility, the three apps vary somewhat in the fidelity of their formatting when sharing with their Microsoft counterparts. Pages and Numbers manage to preserve most formatting reasonably well when opening an Office document or sharing a document with Word and Excel. Numbers tends to break the formatting of more-complex Excel charts, and I typically find myself having to massage the formatting a little whenever I move a chart from Numbers to Excel. Keynote, meanwhile, does not play well at all with Microsoft PowerPoint, and you can generally count on seeing dramatically different results depending on which presentation app you open the file in.
Despite offering several options for sharing files, Pages, Numbers, and Keynote provide paltry cloud integration. You can use iDisk (if you have a MobileMe account) or WebDAV (if you can figure out how to set it up). You can also share files on iWork.com, another of Apple's idiosyncratic attempts at cloud computing. As disappointing as the current cloud options for these three iPad apps are, however, I'm reasonably hopeful that this situation will improve once Apple launches iCloud in the fall.
In contrast to my roundup of Android office apps, which turned up three very strong options, in this group of iPad office apps it wasn't easy to find an all-around winner, because each of the available options is plagued by some pretty significant failings. Apple's apps are a joy to use in their own right, but they lack the level of Microsoft Office and cloud support that serious mobile workers require. The other products have cloud connectivity and Office compatibility to spare, yet they stumble in their interfaces and formatting options.