There are any number of spreadsheet apps available for the iPad, but recently the market changed dramatically when Microsoft released the full Office suite for iPad, including a version of Excel. While you can argue that Excel is many years late to the iPad party (and I wouldn't disagree), the iPad version of Excel is a solid entrant, and instantly changes the landscape for competitive apps.
I've spent quite a bit of time with Excel for iPad, and it's telling that my main complaints have little to do with the actual program: Microsoft has done an excellent job at transferring the power and capabilities of Excel for OS X into Excel for iPad. The program loads quickly, contains more of Excel's core functionality than I would have expected, and the transition from keyboard/mouse to full touch interface has been handled very nicely.
For a company with a history of not doing well with touch (i.e. Office apps on Surface in Windows 8), the interface in Excel for iPad is surprisingly well thought out. The window features six small buttons across the top of the screen, along with five words that are actually tabs on the yes-there's-a-ribbon.
But unlike desktop Excel, where I often find the ribbon too big and too cluttered, the ribbon on Excel for iPad is small and tidy. Each tab (Home, Insert, Formulas, Review, and View) contains only one row of text and icons, and the icons are reasonably sized. The default view opens with the Home tab active, which contains often-used formatting options, such as font face, color, and style; font fill and borders; text alignment; and cell formatting and style drop-downs.
The other ribbons are relatively self-explanatory, except perhaps Review, which is used to scan through comments on worksheet cells. Ironically, Excel for iPad can't actually create cell notes, so the Review ribbon is only used to browse through the existing comments. (You can, however, delete comments; you just can't create them.)
While working on a worksheet, touch actions work as you'd expect them to--two finger drags scroll, and pinch gestures zoom and unzoom. A single tap will select a cell, while a double-tap will open the cell for entry (and display the keyboard). When entering data in cells, you can also use a numeric keyboard, which greatly speeds entering formulas and numbers. External keyboards are also supported, and worked well in my testing--there's even a cheat sheet in the app's help (somewhat hidden in the menu used for saving and duplicating workbooks) that shows you how to navigate and work with the app using an external keyboard.
Cell references can be added to formulas by tapping (and tap-dragging) on other cells as you're working on the formula, further simplifying formula entry.
Once a cell is selected, you can tap-and-drag on one of its corners to expand the selection to a range; you can then move that range (or a cell) with a tap-and-hold operation. You can also bring up a contextual menu of sorts, by tapping on a cell that's already selected (i.e. highlighted on the worksheet). This brings up a bar from which you can cut, copy, clear, fill, or wrap the cell. If you choose Fill, the cell gains small arrows on its right and bottom sides, and you can then drag those arrows to fill adjacent cells, as you can do in the desktop version of Excel.
Microsoft did an incredible job at getting a lot of Excel's power into the iPad version of the app. There are over 400 formulas present--if that's not all of the formulas from the desktop version, it's the vast majority of them. Most of the chart types have also migrated, and creating a chart is as easy as selecting the data to chart, tapping the Insert ribbon, choosing a chart type, and then tap-dragging out a region for the chart.
You can also choose from a large assortment of shapes, add text boxes, and even insert images. However, the image browser is restricted to photos stored on the iOS device; you can't access any media files on your OneDrive, for example. The help system is relatively complete, including a comparison table that shows what you can do in each version of Excel (iOS, OS X, Windows), along with a touch guide that explains how to interact with your data.
From functions to fonts to cell borders to merging cells to table styling to hiding and shuffling worksheets in a workbook, Excel for iPad has most of the features of the desktop version. Some are limited, of course--there aren't nearly as many cell border styles--but there's enough here to meet the needs of even heavy-duty spreadsheet users. I was impressed with the responsiveness of the app, even when working on somewhat larger worksheets--scrolling was smooth, and I didn't notice any slowdowns or other issues.
Things to be aware of
There are some features missing from Excel for iPad. Many people have noted the inability to print--if you want to print your worksheets, you'll need to find a Windows or OS X computer to do so. (You may be able to open a spreadsheet in Numbers and print it that way, too.) This may or may not be a big issue, depending on your workflow.
As noted earlier, you can't create comments; you also can't name cells or ranges (but you can work with existing names), nor create conditional formatting rules. Existing array formulas work, but you can't enter or create new ones. You can see existing sparklines (a full graph in one cell, basically), but you cannot create new ones. Finally, references in formulas to cells on external worksheets won't update.
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.
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.