CHRISWAN  Could significantly improve performance on regularly used Macs

In general if it has been used for about 4-6 months (download, browsing, working with, many larger than, 20MB files) and have less than 30% free HDD space, this app can improve the performance of your Mac

But, as with marriage and tax evasion, your mileage may vary...

I would like to see individual file defragmentation and file/folder exclusion in future versions (it's already there, but you have to do some scripting, which is not my kind of thing, I'm expecting simple checkboxex and stuffs)

Highly recommended  
(Version 1.6.6)

JAKE THE SNAKE  Although the translation below may have been skewed a little, I'd agree with the positive comments.

No other Defrag tools has been as safe and as reliable as iDefrag has been for me over the last few years! Never had any problems or data loss on 5 different drives and 3 different Mac's!

People that claim you don't need to defrag your drive are simply clueless, the Mac OS does NOT defrag very large files over 20 MB's and normally only defrags within the metadata zone mostly. iDefrag goes well beyond where the Mac OS fails. iDefrag not only helps speed up my start up time but it also puts files in order of priority on my drive making them launch a bit quicker too.

And finally, with all disk maintenance tools including iDefrag, it is a wise idea to have a current backup of your crucial files in case anything does go wrong. iDefrag has a safety feature that will verify where and what it is writing during the defragmentation. It may make it run slower, but the peace of mind using it is worth it!

Those of you too cheap to buy this software can simply back up your HD to another drive, reformat your HD and copy everything back using a tool like Carbon Copy Cloner!  
(Version 1.7.2)

SHADOW RAY  I read up on drive fragmentation in OS X and subsequently bought this about a year ago. I've used it on a few different systems and it's always worked smoothly and really sped up startup times for fragmented drives. If your drive has ever gotten to 90% or more full this will likely make a big difference in system speed. I think my system startup time after i first used it was cut by around 5-10 seconds. If you never fill your drives up and aren't working with media drives this will be less useful. I've always run Applejack and backed up before using it but it's never given me a problem or error.

Based on the reviews for Drive Genius, this is definitely the program you want if you're defragging your OS X drives. I was annoyed by the useless demo mode, and the lengthy documentation should be read before using it, but I was very happy upon purchasing fortunately, it's definitely a good buy. I recommend that any media company working on OS X buy this for their media storage drives as well.  
(Version 1.6.6)

REPEATER75  Jan13 is wrong. I ran this on a 200GB firewire boot disc used on a first gen Mac mini 1.42ghz. The difference in speed for system startup and app launch times was very noticeable. I believe this is a utility you only need to run once a year or so, but is well worth $35 for the time you'll save over that year!  
(Version 1.6.6)

MACINTOSH SAUCE  I purchased this program for Leopard, just before the Leopard incompatibility issue notice came out. It's been six months since I have been able to use iDefrag. I defragmented my four HDs on my Mac Pro and everything is running much snappier now. This program is worth the expense IMHO.  
(Version 1.6.6)

DJEFIS  This is a Wonderful Tool.

The only way to mantain the same performances on your Macintosh, after two years with the same installation of the OS X.

Used more than fifty times on my laptops and I never found any problems.

Thanks.  
(Version 1.7.2)

GARY54  I've used this program since I stated using X. Make that about 4 years? Since 10.3? Excellent meat and potatoes program. It does what its supposed to do, doesn't try to do everything under the sun and walk the dog too and its been regularly updated.

It has a good feature set, provides good feedback information about what its doing and the files on your disk. I've used that to clear out gigs of useless junk hiding *what the hell is THAT huge file?*. There is a observable speed up of the system particularly after the first time using it and I would not hesitate to recommend it.

I have always used DW before running a defrag to be sure of the directory being clean ... that only takes a few minutes ...and after to make sure there are no errors after. Cheap insurance which takes but a few more minutes of time.

I have one major complaint about this program. It's .... slow. And I do mean .. slow. Like .. molasses in January kind of slow. If I have a drive with a lot of stuff on it, I set it up to run before going to bed. Most of the time it's done by the time I get up. A few times not.  
(Version 1.7.2)

MAC ADAM  Please update for snowleopard 64bits…  
(Version 1.6.9)

DOM21  What exactly does the "on-line" in "on-line defragmentation" mean? Does it make your hard drive viewable or accessible to Coriolis in any way? If not, then what does it mean?  
(Version 1.6.9)

SHOTGUNWEDDINGS  Where's the iDefrag 1.6.9 demo?

I download from here it is 1.6.8 and I download from Coriolis website same thing.

1.6.8 demo did not work for me. Kept hanging when I launched it. It would go into a endless loop looking for drives.

eMac PPC with 10.5.7  
(Version 1.6.9)