I have used OnyX a couple of times. Some of the tools you can use without much caution. To avoid causing a problem I would advice to use the help feature, to read about what the various checkboxes and buttons do.
I havn't used all the bells and whistles of OnyX, but as I understand, some of its features are safe to use, others can cause a problem, where there is none.
So pop open Help, from the Help-menu, before venturing too deep, and you will be fine.
Not crapware. I've been using OnyX for years and while it does nothing more than what you could do yourself with the proper commands, it's convenient, free, and is supported very well by the developer. As long as you use it in its default mode, it works fine. Certainly a lot safer than "Clean My Mac" which is not only expensive but also sometimes overzealous in what it cleans.
I would imagine many if not most old-school Mac users are using Onyx. I'm guessing OP isn't an old-school Mac user as it'd be hard for me to believe that anyone who has owned a Mac for at least 3-5 years in their lives wouldn't know about it (aside from those who have zero tech-savvy). I've used it for a decade although I only tend to execute the Maintenance scripts (5-10 times a year). If you google the top free apps for OS X, I'd bet that Onyx is mentioned in most of those lists.
As for the safety... While Onyx has sometimes had updated versions (for new OS X) that have had some issues, in my estimation it is regarded as an app that is much safer to use than something like Clean My Mac.