It's inferior to Windows Desktop Search on XP, it's irrelevant on Vista, and OS X has Spotlight.
So the good news is that OS X Google Desktop installation now allows one to omit the search component:
Don't want Google search but still want Gadgets? No problem - The Unofficial Apple Weblog (TUAW)Interesting, but I've never found a widget that was really useful -- Apple's have been pretty disappointing. Worse, Google's directory for widgets is a complete mess. They mix up their own high quality widgets with commercial products and pure junk.
...Guess what? As noted in this comment, a subtle 'Choose your own features' link on Google's download page lets you split up the two functions of GD, and just install the Gadget manager without the desktop search component....
So the main news is that Google may be grudgingly accepting that desktop search is not their cup of tea.