In this piece I am going to give you a list of good references about Excel Online and to a lessor extent, Word, Outlook and Powerpoint Online. For example, this Microsoft page will direct you to a number of videos to help you open Excel files online, something you will work out but will your users ? Very good concise videos
and if you are still in the unsure camp, this article at PCMag shows you pictures of the completely free Ipad version of Excel and gives you mini descriptions underneath.
And if that has whetted your appetite for Online Excel/Office, check out all the rest of the Excel Online hyperlinks on this LinkedIn article I wrote.