Google Spreadsheets is easy to use, free, and works much like every other spreadsheet you've ever worked with. It takes very little time to learn to use it.
You can't lose your work. Once you name the spreadsheet you're working on, or right after you import an XLS file from your computer, Google Spreadsheets saves your file. From that point, every change you make is immediately saved.
The sharing function lets you collaborate with other users (Google account holders only so far). Alternatively, you can invite people to view, but not edit, your work. All changes are live, so you can be talking on the phone and editing the same work at the same time. This is very useful, and it's something you can't easily do with Excel.
There's a good list of mathematical, financial, statistical, and other function types. You won't find many calculations that you can't perform.
The service imports your existing Excel (and CSV) files pretty well. It's not 100%, however: I found that some text formatting (colors, in particular) and date formatting did not import.
Google Spreadsheets does support multi-sheet spreadsheets, just like Excel (I was mistaken in my earlier blog post, where I said it didn't). And here's a nice thing: It doesn't automatically make each file three pages deep, as Excel does, although if you want the extra sheets, it's easy to add them.
There's no print function. But you can export your spreadsheet as a static HTML file.
There are no visualization tools. You can't graph or chart your data. And there's none of the conditional formatting that we'll get with Excel 2007.
Aside from the good collection of formulae, statistical and analysis tools are missing. There are no pivot tables.
If you're accustomed to using the right mouse button in Excel to access cell-specific options, you might not be as productive in Google Spreadsheets, since it has no right-mouse options.
I couldn't find a way to zoom in or out of a spreadsheet. You can change the size of numbers and letters, but not the grid itself. That's frustrating when you're trying to grok a large table.
While the real-time collaboration and chat feature is very cool, it could get confusing with more than two people, because changes made to the spreadsheet are not marked with the person making them.
I'd recommend this service for the new spreadsheet user or the person who just wants to work with numbers or small data tables from time to time. It has functionality most people need, except the critical graphing and printing functions. The collaboration features are just dynamite.
However, Google Spreadsheets only takes you part of the way if you want to use it for serious analysis, or as a tool to create graphics for presentations.
Google PR reps were clear to note that this product is a beta, and an early one at that, so we might see many of the shortcomings addressed soon. Hopefully they'll be fixed before the product is released into open beta. Google would not give a timeframe for that, though.
While you wait, if you are sold or just curious about online spreadsheets, you owe it to yourself to also check out suites from ThinkFree, and Zoho, and the standalone online spreadsheets EditGrid, iRows, and NumSum.