QuickOffice will let you view and edit Excel files, plus it has the ability to access files that are attached to emails (though you have to forward the email to a server first). You can also email the file afterwards.
Dataviz have submitted an update to Apple that will allow viewing and editing of Excel files. (The current price will go up when the update is approved but the update will be free to all who have the current version). Downside is that DocsToGo has a slightly different way of getting at attachments in emails - you need either an Exchange account (though Google now has Exchange ActiveSync, but DTG can't at the moment access it, for some reason that DataViz are looking into). Or you can use their desktop sync mechanism.
Of the two, I prefer Docs To Go, but use QuickOffice when I need to get at email attachments. Hopefully DataViz will sort out the Exchange problem soon.
