Despite the recent and very valid discussion about security issues regarding the Google Desktop Search there is one overwhelming fact to realize: It, the GDS, is as good for your hard drive as the mother-Search is for the Web. There is nothing better, faster and cleaner!
So, what to watch for?
A. Use the Setup to make sure undesired web pages are not being cached. Exclude the sites of your bank, your trades, your frequent flyer club etc. etc.
B. If on a public computer, check or the Google Desktop search. If "GoogleDesktopIndex.exe" is a running process in the Window's Task Manager do NOT use that computer for anything confidential.
As with any truly effective tool, GDS is excellent and a thorough. Be aware of what it may do and don't take any chances.
Go learn, have a try: About Google Desktop Search