It also should be noted that Onyx does not do defragmentation though it is a fine application for other maintenance tasks.
See item #6 on
this macattorney page for yet another cogent discussion of defragmentation and optimization which includes links to some of the articles mentioned by posters above.
FWIW, I have had my Quicksilver since 2001 and do lots of photo and video work, and I have never needed to defrag my hard drive(s) since switching to OSX.
