I bought this program and after testing it out I think it's great. Just to clarify: if almost all of your files less than 20MB (documents, photos, mp3s) then you do not need this program. OSX 10.3 and up will defragment small files automatically, and that will probably be good enough. If however you are like me and you have lots of files over 20MB, and especially if your disk is more than 50% full, then this program is a godsend. In short, it is the perfect utility.

I will assume you are familiar with disk fragmentation and suspect that you need it. If you're unsure, the free demo of iDefrag will let you examine your disk.

Here are a few of iDefrag's features that I liked the most:
- $30 USD - cheap! I did not want to pay $100.
- does every type of defragmenting thing imaginable
- comes with a program to make a boot cd
- can rebuild B-Trees (like DiskWarrior!)

The program includes an optimizer to move applications and file system structures to the start of the disk. The standard defragmenting algorithm (compact) is fast, and there is a "quick" algorithm which just defragments files but not free space. The quick algorithm can be run on the disk you boot from. To run the other algorithms on your main hard drive you need to boot from cd. You can run any algorithm on an external hard drive without rebooting.

The interface is extremely polished and includes a number of nice touches, such as box showing the file associated with each particular part of the disk. The program verifies writes and can be interrupted safely (I tested that on my boot volume). iDefrag respects the "hot area" of the disk and can also rebuild B-Trees, which speeds up the finder along with everything else.

My only complaint about the program is that it can not ignore files on damaged parts of the disk. To be fair, other defrag utilities will not do this either, but it's annoying to encounter bad blocks one at a time and have to reboot each time to delete the offending file. OS X needs a surface scanner.

I highly recommend iDefrag to anyone who works with larger files regularly, and especially to people with external hard drives. It's a great defragmenting program. For best results I suggest the following strategy:

- the first time you run the program do a "Full Defrag"
- after working on large files a lot use "Quick Defrag"
- to free up big blocks of free space use "Compact"
- if Finder is slow defrag metadata only and rebuild all the B-Trees
- running "Optimize" over and over is a waste of time

I hope this review has been helpful. I tested iDefrag 1.5.8 on a 1GHz iBook G4 with 768mb of ram running OS X 10.3.9. It makes a Tiger boot cd so it should obviously be Tiger compatible. Be sure to check out the demo if you're interested. I think this is a great program.