I have to say that I agree with BitWorking on Google Desktop:

This is just like WinFS.

Except that it is shipping today.
And it just works.
And it doesn't require an upgrade to your operating system.
And it doesn't require you to manually tag all your files with meta-data.
And it works outside your "Documents and Settings" folder.
And it's only a 400K download.
But besides that, it's just like WinFS.