There's so much to be said for not passing around an excel spreadsheet, not the least of which is versioning.  Another issue is concurrent edits, which GD handles by allowing multiple users and the ability to lock the document, cell by cell.  That means, yes, you can see one of your colleagues editing the document, more or less live.  Once they're done, you will see the edit fill in the cell, and voila, you have more information that is current.  This is huge for change management requests and collaboration. 

What does Microsoft compete with?

A year or two ago we were all waxxing about how this would end Microsoft.  I don't think that is totally true now, because the benefits of this (free) software probably exceeds the grasp or needs of the average MS Office user.  All the same, it's a great (free) service and kicks the crap out of a filthy, oft-ridden, excel document.  I noticed that MS has been keen to bring Office on to the web.  I tried it out, it didn't work at the time.  I think you might need IE to make it work - that was the case at the time.  I went to their website to find some more info and requirements are not highlighted anywhere.  I did find their community though.  Perhaps these thread titles will shed some light on the Microsoft Experience:

• Re: I can't edit my web pages!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
• Re: Cannot connect to my workspaces during the past 24hrs. - 1200 8/5 to 1600 8/6
• Re: How to I look and add actual html?
• Re: Overly Frustrated

I think we can agree that that's enough about MS.

This is a hot tip

In summary, google docs is a collaborative, portable tool that is most useful to those who work with partners or a team, or anyone that finds they want to access spreadsheets and word processing documents across many computers or a network.  If you want to try it out, I highly encourage you to go to docs.google.com and give it a whirl.  You will need a google account to get going, which is free.  Enjoy and happy productivity!