If you don't need to emulate Microsoft Office applications and want to work exclusively with Apple apps, Numbers, Pages, and Keynote are probably your best bet. They have limited functionality compared to the same apps in the MacOS environment, but the interface is at least similar. And up to a point, you can read, edit, and create MS Office documents without too much of a problem. More importantly, Apple stands behind these products and can be expected to focus on the functionality they provide on the iPad.
On the other hand, if you must work with MS Office documents primarily, you're probably better off with Documents To Go or another of the MS Office emulation products, e.g. Quick Office, Office HD. These products are not perfect emulations for MS Office functionality, either, but they're at least designed with that compatibility as a primary priority.