But the fidelity between iWork and Office is also quite good, as long as the fonts are consistent and the layout complexity is moderate. Frankly, document compatibility and fidelity come from using the commonalities across the apps and platforms, not from any feature inherent to Microsoft's or Apple's apps.
The bottom line is that Office works well enough on the Mac and the iPad as a companion to Office for Windows. But it doesn't work so much better on the iPad that you couldn't use iWork there and Office on the other platforms.
You could also use iWork on the Mac, which is fine as long as you take are of tasks that Office also supports. But iWork offers capabilities, such as really cool presentation builds and multiple-table spreadsheets, that Office does not, so exported files won't work as expected. Ditto in the other direction: Office has capabilities such as page anchors and linked spreadsheets that iWork does not.
The truth is that your workflow is what matters most in your decision to stick to an all-Office app environment or allow a mix of Office-compatible apps.
Outlook and Exchange on OS X, iOS, and Android: Often better to go native than to go Microsoft
The situation is far messier for using Exchange-based applications. I know very few non-Windows users who run Microsoft's Outlook for Mac or OWA for iOS or Android. I don't blame them. They load more slowly (though the latest OS X version is much faster), and they are much clunkier to use than the native OS X, iOS, and Android clients, all of which have strong support for Microsoft's Exchange ActiveSync (EAS) protocol.
Plus, all those native clients let you pull together email, calendars, and contacts from multiple services (while keeping the back ends separate), so you get a fuller view of your messages, schedule, and contacts. Outlook and OWA can't do that, and for employees expected to work anywhere, any time, that inability to see your whole context is a major pain.
Apple's Mail client for OS X also lets you set up out-of-office responses (right-click the Exchange mailbox in Mail to get that option). Other native clients can't do that -- but neither can OWA for iPad, iPhone (which works only on the iPhone 4S or later models), or Android. In those cases, you should use the OWA website, but note that it displays very poorly in mobile browsers and can be hard to use in them. You really need a Windows PC or Mac to set up out-of-office notifications, even when you use Microsoft's mobile clients.
There are a few reasons to occasionally use those Microsoft client apps. The big reason is to access shared Exchange calendars. Only Microsoft's apps -- Outlook and OWA -- can do that in Windows, OS X, iOS, and Android. That requirement is why my company's employees use Google Calendar instead for shared calendars, as we're about 50-50 Mac and Windows; most of us use iOS devices as well. We're not willing to give up the native clients just to access shared Exchange calendars, especially when we can share through a different service (Google, in our case) and access that shared calendar along with the rest of our calendars easiy in our native clients. That's not the officially supported IT solution, but it's the solution that actually delivers in our work-anywhere situation.
Another possible reason to use Outlook or OWA is for delegation of Exchange mail. You can manage that only in Outlook for Windows or Mac -- not in the native OS X, iOS, or Android clients from Apple and Google. Ironically, you can't manage mail delegation in the iPad version of the OWA app, either. More ironically, you can manage schedule delegation in Apple Calendar in OS X, but not in Android nor in Microsoft's OWA mobile clients.
As you can see, in terms of features, there are very few reasons to use Microsoft's communications clients instead of native OS X, iOS, or Android options for functional capabilities.