Excel email attachments can be downloaded, viewed and edited. This is a big plus when traveling with the iPad, or iPhone for that matter.
I have an Excel file in Dropbox that serves as a reference and is password protected. Docs To Go® Premium opens this file and is the reason I purchased it.
The app has 111 functions and while this seems like a lot, one file opens in Read Only mode because several functions are not supported: COUNTIF, TEXT, SUBTOTAL. Not exactly uncommon formulas. I hope they expand the function list in the future.
In addition to online files, you can have local files on your device, which are accessible in iTunes. There’s an option to sync files with your computer, but requires a desktop app (Win & Mac) to sync over Wi-Fi. I haven’t tried this out as of yet.
The non-premium version is available for $9.99 USD, but it doesn’t support file access to Dropbox. Documents To Go® Premium is $14.99 USD and seems pretty steep, but you also get Word and PowerPoint editing, and can view PDF and iWork files.