Even when I have access to Microsoft Office (at work or university) I find myself using Google Docs. I like to use it because I can use it everywhere, I can share the doc easy when I'm doing any project.
Whenever I need any Office I just get a Open Office and it works fine as well, so for me I don't think it's worth it to get Microsoft Office.
I think no. But for PC, best office app is it.
You can use Office with OneDrive account.
For mobile, you can use Google Docs. I prefer Google Docs. This is free and Google Docs has sync-now feature.
If you are working offline, I recommend you using MS Office. Also as there are lot of tools and templates for MS Office though it's completely based on your choice. I use MS Office because I like working with the software running on my own system rather than any third party server.
for basic spreadsheets and word docs, google docs can get the job done. however, if you're doing any complicated formulas or formatting It will fall flat on its face compared to Microsofts offering