While iBooks could be deemed core to the iPad's proposition, following closely behind are Apple's mobile versions of its iWork productivity suite: Pages (word processing), Keynote (presentations) and Numbers (spreadsheets). They are essential to Apple's idea that the iPad isn't just an enlarged mobile device, but a genuine computing platform.
Apple has been successful in this aim, but only to a limited extent. On the whole the three apps are, like iBooks, exceedingly elegant, intuitive and clean applications. Provided your needs are relatively simple and infrequent, at a pinch they might even suffice as your primary port of call for productivity. Likewise, if all you need is to view and make small amendments to already existing documents then they do a job.
There are, however, numerous obstacles in both instances that demonstrate the iPad has a long way to go in this regard. Most fundamental is the patchy reliability of document importing, not only from Microsoft Office docs but also those produced in desktop versions of iWork - where only iWork '09 is supported!
Such issues can often be traced to the limited font support on the iPad, but there are also numerous features that aren't supported in the mobile versions. One classic example is in Pages, which doesn't support endnotes or footnotes, while it's the only one of the three apps that can export to MS Office documents - others must be converted using the desktop versions of the applications! Another irritating omission includes the lack of a presenter view in Keynote, making the iPad largely useless for making serious presentations. Getting documents to and from the iPad is far too laborious an exercise, too, as it's far too reliant on iTunes and iWork.com
The one saving grace is that typing on the iPad is largely very easy. When held in two hands it might be an inch or so too large to type really quickly, but set it down in landscape mode you can type accurately and at reasonable speed. It's certainly good enough for the occasional document and regular emails. You can even use a Bluetooth keyboard if you wish, though by that point you may as well get a laptop out and use that instead.
It's not enough to prevent the great majority finding the iWork apps frustrating experiences, though. They certainly won't allow you to leave your laptop at home every now again, reinforcing the view that the iPad is stringently a companion device. Ultimately, while they're fun to use, none of the iWork apps are that practical. Fun may sound like a neat idea, but practicality is what they ought to be about.