The app itself is free, but there are two caveats. First, there isn't a native iPad version. You can, of course, use the app in the horrific 2x mode that you can use for iPhone apps on the iPad, but that is not a good solution. Second, it requires an Office 365 subscription.
An Office 365 subscription for the home costs $99.99 annually, or $9.99 per month. That's not cheap, although compared to buying a standalone version of Office Home and Student 2013, it's a better price.
It is, however, annual, and must be renewed annually to continue to use the product. In addition to access to Office on iOS, a single subscription covers five PCs, Macs or Windows tablets, and comes with with Word, Excel, PowerPoint, Outlook, OneNote, Publisher, Access, 20 GB of SkyDrive storage, 60 global minutes per month on Skype, as well as the ability to stream full versions of Office applications to any Internet-connected PC running Windows 7 or Windows 8 using Office on Demand.
Microsoft says the app is its “official Office companion optimized for your iPhone.” It allows Word, Excel and PowerPoint editing -- yes, editing -- viewing, and creation. Documents are stored on SkyDrive, SkyDrive Pro or Sharepoint in the cloud, which means Office 365 subscribers will be able to easily access recently edited documents when they reach their the desktop or Windows tablet.
There if offline funcdtionality as well -- although an Office 365 subscription is required, users can edit and view recently used documents when disconnected. Once the device reconnects to the Internet, any modifications made to offline documents will be saved to the cloud.
Recently edited documents will be synced, so that if -- for example -- a desktop PC or Surface tablet edits a document, that file will be displayed in iOS through the recent documents panel.
Hopefully an iPad version is coming soon, as document editing on a small iPhone screen isn't precisely what users have been calling for. Users need an iPhone 4 or later, or a 5th generation iPod touch, running iOS 6.1 or higher, to run the app.
When some reports of an upcoming iPad version of Office appeared last year, an Android version was also rumored. No mention of an Android version has appeared yet, of course, but considering Android's dominance in the world, it should be coming, too.
Office Mobile for Office 365 subscribers is currently U.S.-only, though it will expand, naturally.
A slideshow of screenshots is available here.