See, OS X isn't as bad as windows when it comes to the "unwanted" files, registry errors and that kind of thing, so the "cleaning" apps don't have nearly as much work to do.
OS X runs its own daily, weekly and monthly maintenance scripts anyway (which can be run manually from Onyx too) and the file-system is self-defragging, so that's another major problem out of the way. the only regular maintenance a Mac needs is to Repair Permissions every few weeks.
The only other thing I use Onyx for is to change the Dock to 2D mode to get rid of that dumb reflective look.