You said it! Ozzy is a horrible singer, but he is always backed up by a top-notch guitar player (Tony Iommi, Randy Rhoads, Jake E. Lee, Zakk Wylde, etc.). IMHO, other shock artists like Alice Cooper, KISS and Twisted Sister are 70% this-will-freak-out-your-folks and 30% band.
Manson? His ratio is 90%/10%. There is absolutely NOTHING there but hype and gore.
And quite a few cover songs.