MMCSHANE63  Nicolasd, I read your comment and I think that "useless" might be a bit harsh (unless it destroys all of your files during a run and it didn't for me). However, your comment about rock solidity got me thinking and I can't remember any app that I would describe as that. Can you suggest one that you would describe as RS? I can tell you (see other posting) that it worked for me and made a significant (good) difference.  
(Version 1.6.5)

MMCSHANE63  Having read a number of reviews here, and elsewhere I bought this and have now used it on my Intel Core2 duo, 20 inch iMac in target disk mode (from a MacBook) and also on an external FW Drive (LaCie). My conclusion is that it works and it makes a significant difference. It does not seem to have made much difference to startup, but apps open considerably faster. I store large(ish) video projects on the external drive and this has to be worth it.

Full iMac defrag in approx 9 hours. Full external HD defrag in approx six hours. I'll be using this regularly.   
(Version 1.6.5)

MMCSHANE63  Great proof of concept. The developer was very quick to respond to my email and courteous with it.

I'm using it with an Intel CD, 2G in conjunction with a G3 iMac or a G4 eMac or an Intel MacBook. Apart from fairly sparse instructions I don't think ir gets much better.

mmcshane63  
(Version 0.92)