Tried this on my normally smooth running powerbook.
Initially used fsck/Disk Util to ensure file system and permissions all OK.
Then tried iDefrag on that drive (by booting it into firewire mode and running iDefrag from my main iMac).

Halfway through a full defrag, iDefrag states "cannot continue - file system not modified" (or similar) for some reason.
Try another algorithm (compact).
Halfway through, iDefrag again states "cannot continue - not modified" for some reason.
Try another algorithm (metadata).
Halfway through, iDefrag again states "cannot continue - not modified" for some reason.
Try another algorithm (optimise).
Halfway through, iDefrag yet again states "cannot continue - not modified" for some reason.

Have now gotten VERY worried (as no algorithm appears to be completing) so have given up and tried to restart the OS ----- no go there.
Use DiskWarrior to repair the damage done by iDefrag, OS now boots (phew) but Quicktime no longer works :-x
Spend AGES looking for a Quicktime installer (standalone) to over-write and re-install it ------ Quicktime finally works properly again.
I don't think this was a problem with QT itself, as it was working fine before (as was the OS). iDefrag simply "broke" some apps/the OS (repaired by DW) and the un-repairable one (just happened to be QT) had to be re-installed.

I now have a working powerbook again but am VERY wary of ever letting iDefrag touch any other HD of mine (or anyone elses) from now on.

To top it all, the study pb is actually slightly slower to boot now after the debacle with iDefrag.

If you want to use it use with caution.

From Developer:

iDefrag respects the "Hot File" zone so it will not slow down OS X.

OS X's automatic defragmentation is good, but has limitations; it won't defragment free space, files larger than 20 MB or files with fewer than 8 fragments.

Also, iDefrag will optmize your hard drive, moving OS files close together, which can make a difference.

Of course, your mileage will vary. See our web-site for more information.

For anyone who works with large files, iDefrag is extremely helpful, as OS X only defragments files less than 20MB in size. Any type of digital media editing setup (garageband, logic, final cut) would benefit from defragmenting, as well as P2P...

this is the beta 2 release. 1.5 is not ready. and as far as "them"s comment is concerned, iDefrag is the only mac os defrag too that recognizes and optimizes the hot cluster itself, always leaving that space available for hfs+ journaling features to do their thing. if you visit the website you can read more about it

OS X and HFS+ automatically defragment files and the do adaptive file clustering where commonly accessed files are palced in the faster parts of the hard drive.

So chances are iDefrag will slow down OS X.