I signed up to be a beta tester for Google Spreadsheets the other day and yesterday I found an invite in my gmail inbox.
Overall I would say it is pretty nice. The syntax, equations, and functionality of the spreadsheet were similar to Excel and OpenOffice Calc. It was pretty responsive for being a web application. As other people have noted, the biggest lacking feature is graphing.
You can load a file from your computer, save your work to your computer, or save your file online with Google. This was kind of interesting because once I had named and saved my file the first time, there was no "save" button (it was grayed out), it just automatically saved as I worked. I did have a formatting issue where I tried to change a few cells and the menu was there but I couldn't get the numbers to change. I'm sure they'll get this figured out.
This was kind of neat although at first I didn't see much utility because OpenOffice Calc is free. However, having 3 different computers that I use regularly I can see the benefit of saving online so that you can use your work anywhere there is a computer and an internet connection. Also the sharing feature of letting other people use your files seems like it could be useful.