iWork.com is a convenient way to share iWork documents, particularly with people who don't run iWork themselves - which will be useful in a crossplatform environment.
You can use iWork.com to share documents, spreadsheets, and presentations from the iWork '09 productivity suite. But the people with whom you share those documents can only view, download, and comment on them; they can't actually edit them.
All that said, iWork.com is a convenient way to share iWork documents, particularly with people who don't run iWork themselves - which will be useful in a crossplatform environment.
See also: Apple iWork '08 review.
iWork.com: how it works
Each of the three iWork '09 apps - Pages '09, Numbers '09, and Keynote '09 - now have an iWork.com button on their toolbars (as well as a Share via iWork.com menu item). Press on that button from any active document, and you'll be asked to enter the email addresses of the people you want to share it with. (Addresses are autofilled from Mail's contacts.)
You can add an optional message, and specify whether the people you're sharing with can comment on the document, download it, or both. If you click on the Advanced button, you can specify which file formats iWork will upload: in Numbers, for example, you can specify Numbers '09, Numbers '08, PDF, and Excel.
When you then click on the Share button, you'll get a status message tracking the upload. When it's done, you can either click on OK to get back to editing your document, or View Document Now, to open the document on iWork.com in your default browser. The recipients on your "To" list will get an email informing them of the share. When they follow the link from that message, they'll also see your document in their browser.
When viewing an online document, you see an Add Comment at the top of the screen; you or your sharers can highlight some text, click that button, and then add a comment about the selected text. Sharers can also leave notes about the document in general, which everyone else can see. A navigator sidebar offers thumbnails of each page, and a Download drop-down menu lets you choose which file format you want to download to your machine.
To comment on a presentation, spreadsheet, or document in iWork.com, highlight some text and click on the Add Comment on the top of the screen.
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