Actually I wouldn't go as far as to say Onyx is safe to use. It is not a maintenance utility despite others calling it, Cocktail, Macaroni, Applejack, Jaguar-Tiger Cache Cleaners, and other multifunction utilities that do more than one task. The only safe utility to use is a backup utility. Everything else should be treated as an after backup utility used only, and I repeat only for troubleshooting, when you know what it is you are troubleshooting, and are ready to restore the backup if the troubleshooting doesn't save you time. If you don't have a backup, some troubleshooting utilities can actually dig you a deeper hole, whereby data recovery is the only way to restore your situation with expensive data recovery companies. Moral of the story, backup first, worry about alternate utilities later. My backup FAQ* goes into detail on that:
http://www.macmaps.com/backup.html
The real danger with many of the other utilities which specifically empty system caches is that caches can get corrupt, sometimes requiring a complete erase and install to resolve a persistently corrupting cache file. The real danger of prebinding utilities is you can render your Mac unbootable:
http://www.unsanity.org/archives/macos_x/shock_andawe.php
Permissions sometimes is not repaired properly by these utilities, unless you use Disk Utility of the boot drive. Though thankfully with Tiger and newer at least you can use the installer disc's disk utility on a newer version of the operating system than is on the installer DVD. Previous versions of the operating system were not as generous, and required you have an equal or greater version of Disk Utility to do repair permissions.
Whoever recommended Onyx did so only because they were lucky or knew what they were doing, and assumed you would too. These swiss army knife utilities need to be guarded with care, and only to be used when you have a backup on standby.
- * Links to my pages may give me compensation.