One last limitation is that Excel for iPad can only be used on one workbook at a time. If you have two worksheets you want to work on, and they're in different workbooks, you'll have to close one to work on the other. Usually, this isn't a problem, but it can get annoying if you're working on related workbooks and need to pop back and forth a lot. (Have a second iPad? It'd come in handy right about now.)
By default, Excel for iPad constantly saves your work — so if you're working on a mission-critical workbook, you'll want to duplicate it first, just in case you do something bad. (You can also disable auto-save in the same menu you use to duplicate workbooks.)
Files can be saved to the iPad (and transferred in/out via iTunes' Apps section), or saved to your OneDrive cloud disk. A nicely-laid-out file organizer lets you browse files on both the iPad and on OneDrive, although there's no file preview functionality — if you want to know what's in a certain file, you'll have to open it.
Excel doesn't support Google Drive or Dropbox, which isn't surprising — you won't find support for OneDrive (or Dropbox) in Google Sheets or Numbers. The lack of support for other cloud services is troubling, but not crippling — Excel, for instance, shows as a supported app in the Open In menu in Dropbox.
The elephant in the room
The elephant, of course, is the Office 365 subscription model that Microsoft applied to Excel for iPad. While the app itself is free to use (as a spreadsheet viewer), to actually edit workbooks, you need an Office 365 subscription, which will set you back $100 per year. For that, you get 20GB of OneDrive cloud storage, and the ability to use all of Office on up to five computers (OS X or Windows) and five tablets.
That $100 is a lot of money — or it's not a lot of money, depending on your needs and perspective. If you're not a heavy spreadsheet user, and you're perfectly happy using Office 2008 on your Mac, then you might view $100 per year as outrageously expensive. But if you live and breathe spreadsheets, own two Macs, a Windows machine, two iPads, and travel a lot, you may see that same $100 a year as a bargain — because the cost of buying and installing Office (it also includes Word and PowerPoint) on the Macs and Windows machines would quickly cover at least five years' of the subscription price.
Each user will have to decide if they feel the price is worth it, based on their individual use patterns.
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