As it stands, the only way to open Microsoft Office apps on an iPad and guarantee document fidelity is to use some sort of remote access system to run Office on a PC running Windows. There are a couple of remote Windows solutions out there but CloudOn has just been updated and looks pretty good.
CloudOn's approach is different to many others and addresses one of the chief criticisms of iOS. Rather that try to replicate the entire Windows desktop on an iPad - something that leads to frustration as Windows was never really designed to run on a touchscreen - CloudOn starts by providing the iPad with a filesystem.
Using DropBox or box.net, CloudOn makes it easy to access your documents. When you open a document, you're in Office 2010 without having to manage your way through Windows Explorer with your fingers.
CloudOn is free.