OpenOffice Advantages

The thing I love about OpenOffice is that I can install it anywhere I want, as many times as I want, on any platform I want (well, as long as it's Windows, Linux, or Mac) for absolutely nothing.

Depending on how you purchased MSOffice, you may have licensing restrictions on installing on more than one computer. OpenOffice has no restrictions whatsover.

OpenOffice...95% of the functionality of MS Office, 0% of the cost!

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Eric Imboden
http://commoncontext.com
my take on tech : http://redsquiggly.com

There are a number of advantages.  The top 2 would be...
1, it is standards complient, therefore your documents should be editable in a wider range of alternative applications.
2, It is FREE, this one is a biggy as Office is not cheap.