Opening an Excel file that's has a lot of design formatting done on it, never opens right. Columns not wide enough, label boxes in the wrong place, hyperlink buttons in the wrong place, colours completely different to the Excel ones, non-standard fonts... all wrong.
It's the same the other way around, if you open an Open Office created sheet in Excel
If it's not a bog standard and simple spreadsheet, it'll annoy you all the time you use it.
If you never do any visual design on your sheets, you might think it OK. But then it is free, so try it out for yourself.