Best Microsoft Office in-the-cloud service: CloudOn
Running Windows on an iPad can be a disconcerting experience, given how they weren't designed for each other. But the CloudOn app nicely accesses the 2010 versions of Excel, PowerPoint, and Word, as well as Adobe Reader, hosted on Windows Server. Plus, it supports the iPad's native keyboard and Share facility. And it uses your existing Dropbox, Box, Google Drive, and SkyDrive cloud storage for files, so you don't have to worry about version control across multiple devices.
It has a few flaws (such as no AirPrint printing) but is a very handy option -- as long as you have a live Internet connection.