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.