I'm still waiting for my Google Spreadsheet "limited test" invitation to arrive, but a few lucky punters have received invitations already. One is CNET's Rafe Needleman. He lists some pros and cons, which I'll summarize here:
Pros: easy-to-use; free; auto-save; sharing function; lots of functions available ("You won't find many calculations that you can't perform"); imports Excel and CSV files "pretty well"; supports multisheet spreadsheets.
Cons: no print function; no visualization tools (charts, graphs); no "conditional formatting"; no statistical and analysis tools; no pivot tables; no right-mouse options; no zoom; real-time collaboration and chat feature is cool but potentially confusing.
Overall Rafe recommends it, but notes that "Google Spreadsheets takes you only part of the way if you want to use it for serious analysis or as a tool to create graphics for presentations." Philipp Lenssen also has an early review and he seems generally impressed, but also notes the lack of advanced functionality.
Even though I've not had the opportunity to play with Google Spreadsheets yet, early reviews indicate that it is (as expected) a relatively simple tool - with few of the complex functionality that Excel has. That's precisely why I warned in my initial analysis not to expect an 'Excel killer', although Ed Burnette couldn't help himself! :-) Ed has redeemed himself though by posting an Image Gallery of Google Spreadsheets up close, plus a couple of videos of the app in action: