I am signed up for the Google spreadsheet early trial. The one test I did for it - upload an Excel worksheet, make some edits, send it to someone else through Google - worked as I expected.
The strength of this offering is the very simple sharing model it enables; you can have any arbitrary group of people very easily share access read only or read write to a worksheet.
The biggest weakness is that it's slow - really slow - at least on my Firefox / iBook G4 combo. Serious spreadsheet junkies will want a local client, at least for the bulk of their data entry and edits. I'd hate to do a lot of typing in it.
Disclaimer: I am a Socialtext shareholder and founder.