I'm one of those that swears that defrag does help. Not so much for speed but the fact that the computer seem to run cooler when it doesn't have to scuff around the harddrive so much.
Although there's really no good tools yet for Mac OS X to defragment the harddrive, it's a long process (several hours). The only one so far is Drive 10 from Micromat, but they are going to release TechTool Pro 4 very shortly and the KING of disk maintenance, Alsoft, is releasing version 3 of DiskWarrior (however without their optimize tool, that'll come later).