How important is security?
While security is an important issue, for all of the documents that I or my company create on Google Docs and Spreadsheets, I'm not really worried if someone else sees them.
The three main cases in which they can be seen is if
(a) someone gets my password (I take care over where I type this in, and it's a random alphanumeric string that only I know),
(b) if a Google employee decides to have a look (I think that they have better things to do with their time, and I'm sure there's a lot more interesting Google Docs out there) or
(c) if Google are ordered to part with the info by law (if it comes to this, I probably have more pressing problems, and "I've got nothing to hide" etc).
I think that it would be difficult for malicious outsiders to get hold of the info if you're careful about passwords, ownership etc and for me at least, the benefits (proper concurrent spreadsheets and docs) outweigh the fairly small risks. I think that the majority of users will be in the same boat as me. For some, any risk is unacceptable, and these users will naturally have to fall back on more secure and less accessible options.