Google has gone to great lengths to improve fidelity with Microsoft Office, but it hasn't gone far enough. Google Apps can capably open and work with Microsoft Office file formats, but many features--such as tables of contents, footnotes, or inserted images--end up being reformatted in Google terms, and they remain that way when you revert to the native Microsoft Office software.
Zoho Docs is in the same boat as Google Apps in regard to file fidelity, but Zoho has an advantage over Google in supported file types. Zoho can export files in the current XML-based formats used in Office 2007 and 2010, while Google Apps is limited to saving Office files in the outdated .doc, .xls, and .ppt formats.
As you might expect, Office 365 beats out both rivals in this department. Within Office 365, you won't necessarily be able to insert or edit many of the advanced formatting features from the desktop Office programs--such as footnotes, headers, or a table of contents--but you can view them, and at least they won't screw up.
Winner: Office 365 Microsoft's online offering takes this category by a clear margin.
Mobile and Browsers
Looking at performance on mobile devices and Web browsers, it should come as no surprise that Office 365 works best with Windows Phone 7 and Internet Explorer 9, while the Google Apps package excels on Android and in Chrome. Zoho doesn't have the mobile platform or Web browser loyalties of the other two.
With the upcoming "Mango" update, officially named Windows Phone 7.5, the Microsoft mobile platform will have native integration of Office 365 and Windows Live SkyDrive, making it the best integrated mobile platform--assuming that you use Office 365 and SkyDrive.
On the other hand, if you use Google Apps, Android is the mobile platform for you. Google offers a Google Docs app for Android; the app is a little rough around the edges, but for many tasks it works better than using the mobile browser, especially on a smartphone.
Office 365 and Google Apps both work fine for viewing files on an iPhone or iPad, but editing is another story. In Office 365, the file opens in a browser-based viewer; then, if you want to edit the material, you have to use the iOS 'Open In...' function to open the file in an iOS app such as Pages or Documents To Go.