Put me in the LibreOffice camp. I've been using it since early high school when it was OpenOffice.org.
Bonus tips:
1. If you're going between computers and you need to present or print something, use a PDF, not the document file itself.
The entire point of the PDF format is that it's
portable (that's what the 'P' means). Avoid looking like a total amateur at presentations while you try and monkey with powerpoint compatibility, or be sure that your typesetting work transfers
exactly on complicated documents like a resume, especially if you're a university student and you're sharing any sort of academic interaction with me. PDF is an incredible format, and it will always do right by you, so long as you don't need to edit on your destination computer.
2. If LibreOffice just won't open something right, it could be that it's not interpreting the MS office file format correctly, or it could by that it is and the Microsoft program isn't (sadly, this happens). If so, and you're running windows, check out programs like
Powerpoint Viewer, a read-only Powerpoint program by Microsoft. (Only a limited capability to do this
exists with the notoriously-incompatible Access, which in my opinion is the single worst piece of software ever written. Worse than Stuxnet.)