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.
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. 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.