I use LibreOffice, and I've never had a problem opening, writing, and sharing MS Office files with Office users. Supposedly incompatibilities exist, but in however many years of everyday use, I've never encountered a problem.
In fact, at my previous job I used LibreOffice to create the MS Office templates for standard company documents (letterhead, invoices, etc.) because it was far easier to create consistent layouts. (By contrast, Excel still employed some weird, subjective column-width calculation at the time that involved numbers of characters in the default font).
Office can now read and save to OpenOffice/LibreOffice's file formats, which means Microsoft expects that a good number of their users have to share documents with OO/LO users.