Google Spreadsheet is, if nothing else, a very neat experiment. The UI is very slick. Some of the collaboration features are neat. But you have to wonder why Google is doing this. Do they really think they can compete with a desktop office suite?
Office is not dying and it will never be replaced by something web-based anytime soon. This is true for a number of reasons: enterprises, the key drivers of this market, aren't going to go for it; it requires a LOT of trust for sensitive, confidential stuff; for publicly traded companies, there's the compliance and regulatory issues; and perhaps most important, we don't (yet) have truly ubiquitous access.
Jeff discussed whether Google Spreadsheet will erode marketshare for the casual Excel user, and mentions that "[t]he perspective that gets lost in these debates is that Microsoft itself appears to be well along the way to delivering componentized Office (Live) that satisfies the needs of casual and advanced users alike." I disagree with the idea that these so-called "Office 2.0" products will replace desktop offerings.
I think Jason nails it - these are Google's attempt at a preemptive strike. Instead of battling Microsoft on their own turf (search and ad revenue), they want to shift the battle to Microsoft's turf (Office and OS). From that perspective, even half-baked solutions like the Google Pack and mere speculation that Google will launch an OS is enough to make Microsoft notice.
As I've said many times in the past, web-based tools will not replace desktop tools - but they will supplement them. In my mind, this is the real meaning of Web 2.0: ubiquitous access to your data in the richest format available.
Microsoft has innovated a lot with Office 2007, but the threat of a battle with Google in that space will force them to focus more on Sharepoint and online integration - at the expense, somewhat, of focusing on search and online advertising.