TIMBARNES  After a series of problems with my Mac Pro (2008), I was forced to take my machine in to the local Apple store, where they re-installed the operating system. This helped, but did not solve all the problems. I found the machine was running very slowly, and there seemed to be a lot more disk activity than I was aware of. I had previously used the Disk Utility (it saw problems but could not fix them), and TechTool Deluxe (it made changes, but didn't solve the problems), I finally purchased DiskWarrior.

DiskWarrior was able to re-build the file system, and since using it my machine has been back to its original high levels of performance.

I don't know how it works, but in my experience, DiskWarrior was able to fix problems that other utilities could not. For that reason it receives my highest recommendation.  
(Version 4.1.1)

TIMBARNES  Any software that deals with the low-level structure of your computer's file system has to be 1000% robust. Errors and crashes are an absolute no-no for these kinds of utilities.

Unfortunately, I have had a number of hangs while using iDefrag's latest version, and I've also seen file system corruption that has required me to run utilities like DiskWarrior to repair the file system.

So while I think there may well be a case for doing defragmentation from time to time, I can't recommend this application.  
(Version 1.6.9)