Google's online spreadsheet is going into limited beta today. As of this writing, nobody I know has actually used with the service, however screenshots are up on Google's site, and we can tell a few things.
For one, this is no competitor to Excel, either the current version or the Excel we'll see in Office 2007. The simple interface reminds me more of a very early Excel, or Lotus 1-2-3, than a modern spreadsheet. For people who occasionally need to compute a grid of results, it looks like it will be extremely useful, but for spreadsheet jockeys who need Excel's multiple-page calculations, cross-tab features, and programmability, it looks like Google will come up short.
However, as a tool for collaboration, Google Spreadsheets is going to walk all over Excel. Google will have a built-in chat client and allow simultaneous editing of a sheet. This will allow two (or more) people to put their heads together on numbers even if they are not sitting next to each other. Also, since Google Spreadsheets will save your work on its own servers, you won't have to worry about sending your file around to other people -- any authorized user will be able to pull it up online. The online storage may dissuade people who want to use the product to work on sensitive financial data, though.
Google Spreadsheets is much like Writely, Google's online word processor. Functionally it's simple and does not compete with the rich feature set of existing traditional software. However, it uses the Web to offer something that software cannot: great collaboration features and Web-based file storage. Also, it's free. So while neither Writely nor Google Spreadsheets are likely to woo professional users of Microsoft Office, more casual users may find the tools very useful, and certainly more cost-effective.
We will update this opinion as we learn more.