[hatcravat]

There isn't really a substitute for Excel if you want to use big spreadsheets (more than, say, a few hundred rows and a few dozen columns). Neither LibreOffice nor MS Office is really good for hard-core curve fitting, but Excel has a few options that LibreOffice lacks (at least nth-degree polynomial fits and maybe some others). Even when you can use LibreOffice for your spreadsheets, it's often significantly slower.

LibreOffice works just find for my word-processing needs.

Edit: Forgot about Gnumeric for a moment. It's probably twice as good as LibreOffice for what I need, but doesn't do 100% of what Excel does.