The iPad is great for many things, but it's no replacement for a Mac or Windows PC. If you're traveling with an iPad--but no PC--sometimes you need Windows 7 and Microsoft Office, particularly if you're running Windows at work.
OnLive Desktop is a free app (yes, free) that brings a cloud-based Windows desktop, plus Microsoft Word, Excel, and PowerPoint, to your iPad. The Standard version includes 2 GB of cloud storage, which appears as your Documents folder on the Onlive Desktop display. The Pro version starts at $10 per month, has 50 GB of cloud storage, and can add additional PC applications. Versions for Android and other platforms are coming soon.