No duh. It's free instead of $400 or whatever MS wants for office these days and it works for the vast majority of what people want to do (write a document, create a basic spreadsheet).

How many people are going to continue to shell out $400 for a word processor and spreadsheet program for there new $1000 laptop they bought there kids? Not many when the realize a reliable free alternative is available and it lets them open the word files people send them in e-mail.

I would say the only advantage MS has is there are so many spreadsheets people have made that make extensive use of macros and VB, which open office doesn't support.