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I have used it since 10.4. Works great, better than most paid apps out there.
 
used it quite a bit,

as far as it's usefulness, it's debatable, I think it's claims of tidying up your disk and such like is exaggerated.
 
I've used OnyX for a long time and completely love it. It consistently saves Macs I encounter from slowness. Especially after a major system upgrade — for example, a lot of the sluggishness in Mavericks was fixed after I ran OnyX. No clean install necessary.

I wouldn't necessarily say it saves hard drive space. It's just a GUI frontend to a bunch of utilities that clear caches, verify your hard drive, repair permissions, clear out corrupted preference files, and a bunch of other stuff that you may or may not want to do. I avoid apps that claim to "clean" your drive; they typically clean too much.
 
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