Excel for iPad is an excellent iOS spreadsheet app. The touch UI is well thought out, and support for the full set of functions from the desktop apps means you can work on even complicated workbooks while away from your computer. Saving files on OneDrive means they're in sync across all your devices, and available wherever you go.
The main downside for casual users is the $100 per year price, which is expensive for those who don't often need the full power of Excel on their iOS devices. For these users, there are much less expensive options (though none as fully-featured as Excel for iPad). For serious users, the $100 per year cost probably isn't an issue, because of the number of machines and devices it covers, and the fact that it insures they'll always get the latest and greatest version of the desktop and iOS apps.
Irrespective of price, though, Excel for iPad is the best spreadsheet app I've yet seen on the iPad.
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