LibreOffice is actually pretty great, it's not "run on a Pentium 100 MHz" fast but it's stable and works well for basic office spreadsheet/word processing/powerpoint tasks.
It works well until you need to send and receive documents from MS Office organizations. LibreOffice mangles layouts and formatting in DOCX and PPTX files.
The UX is mighty rough. Last I worked heavily on Calc it wasn't terribly compatible with other spreadsheet software like gnumeric, though perhaps that has changed.
