Existing users, 
I would highly recommend reading this:
http://www.rixstep.com/4/2/onyx141,00.html
Refers to an older version, but I downloaded this version (1.6.4) and the same things apply...
OnyX
Maintenance, optimization and system personalization tool.
Version: 2.1.1
Before you use this app...
Feedback Type: Review
Contributed by: tubadood Saturday, October 22 2005 @ 11:15 AM PDT
Product Platform: MacOSX
Used Product For: Less than a month
Recommend Product: NO
Overall Rating:
Support:
Features:
Quality / Stability:
Comments
Before you use this app... - Cortig
The article on the page you site does not justify not using or not installing the app IMVHO. It only says what the author doesn't like in design or way the app is setup. It doesn't cite any major bug or critical flaw in the app. The fact that the app uses an Apple installer requiring admin password is fairly common nowadays. The fact it uses the Apple Help shows it's inttegrated in the System (now if Apple could make this Help snappier...:-\ ). The use of brushed-metal is a discutable choice, but it's just a matter of taste. Nothing really serious.Saturday, October 29 2005 @ 10:32 AM PDT
Before you use this app... - tuqqer
That is the silliest, uninformed, most paranoid article I've read all year.Onyx is one of those perfect programs for the Mac.
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Saturday, October 29 2005 @ 10:19 AM PDT