There are many apps that let you open and edit Microsoft Office (Word, Excel, and PowerPoint) files on an iPad. I use Documents to Go Premium mostly because I have been using a version of this app since I carried a Palm OS device in the 90's, but there are several other choices. As handy as they are, these apps do not provide the entire Office feature set. For example, I often use PowerPoint files with embedded audio or video or complex transitions when I teach at CLE seminars or law schools. For the most part, those advanced features are lost when I open a PowerPoint file in one of the many iPad apps that allow you to open, create, and edit basic Office files. Also, key features in Word such as the all-important Tracked Changes don't work with most iPad apps that claim MS Office file compatibility. What if you want (need?) to use your iPad as full-featured tool to create and edit Office files?
No matter your choice of apps, using an external Bluetooth keyboard makes creating and editing Word files and responding to email much easier. If you don't mind adding bulk to your iPad in favor of the convenience of having a keyboard with you at all times, consider a keyboard case (aka folio) such as those made by Logitech, Belkin, Targus, or Zagg. My personal preference is a standalone Bluetooth keyboard you carry only when you need it. A standalone keyboard avoids spoiling the slim profile and relatively light weight of the iPad the majority of the time that an external keyboard is not required The most popular standalone keyboard is Apple's own wireless keyboard, but there are alternatives from Logitech, Targus, and Genius.
Read more of Scott's review on his Affinity Blog here.