This review of Google Spreadsheets is now outdated.
Import
Google Spreadsheets requires you to be logged into a Google account. If you don't have one, it will prompt you to create one. You can import spreadsheets from Excel or any other standard .xls or .csv file or you can create a spreadsheet on the web and download it as an .xls or .csv file

Screen Capture of GoogleUpload a New Version
Google Spreadsheets seems to be geared towards sharing a spreadsheet, but maintaining a backup copy on the desktop. This is a wise course of action with experimental new software, anyway, but Google offers a handy Upload new version option on the file menu to facilitate doing just that. Spreadsheets also now supports versions, so it isn't as risky to simply leave a document on the Web for editing and publishing. This feature has changed.
Limitations

Screen Capture of GoogleGoogle Spreadsheets is obviously in the experimental stages of development, which means you may see periodic messages that the service has been disrupted.
There is no way to copy and paste a spreadsheet from Google into Excel, although you can export and import some spreadsheets. This may cause formatting problems. There's also no way to directly embed content from a spreadsheet into a Google Docs document.
The Bottom Line
Google Spreadsheets may still be new, but it's a useful tool for sharing small spreadsheets, and it has promise of getting even better.