Google Spreadsheets is out and, I must say, it's a pleasure to use. I just got my invite, and I've been fiddling with it for a while.
Go to spreadsheet.google.com and sign-in (if you've been invited to the Limited Beta). It loads extremely quickly, in my case taking only 5 seconds to load. Once loaded, you can barely tell whether you're using a desktop application or something through the internet.
The interface is clean and simlple; it's also quite similar to Excel, just as you would expect in any spreadsheet program. All in all, the interface looks like a Googlified version of Excel. Quite pleasing to the eye.
Google Spreadsheet incorporates all basic features that you'd expect in a spreadsheet program. You can even use formulas and sorting. Files can be downloaded in .xls and .cvs format, and can also be uploaded from ones computer. Cells can be merged, basic formatting can be applied, etc.
Nothing revolutionary so far? That's because I've left the best for the end. Here it comes: You can allow multiple people to view/edit files created in Google Spreadsheets. Files created in it are accessible through the internet, from any location. All changes made are saved in real-time, so you needn't worry about losing your data in case you accidentally close the window, or your browser crashes.
The part that I like best about Google Spreadsheet is that files created in it can be accessed from anywhere, by as many people as required. Basically, if I'm in Ohio when I create a file, and I go to New York, I can still access my file through the internet. No need for burning discs or carrying flash drives. Simply amazing.
Although this marks Google's foray into Microsoft's core business, Google Spreadsheet is unlikely to pose much of a threat to Microsoft Excel, as most features which make Excel so popular are missing in Spreadsheet. For instance, the much-used and loved (or hated) macros and graphs are not available in Spreadsheet.
Given this, and the fact that Spreadsheet is free (and cool), the two are likely to co-exist rather than interfere with each other.