Manson is an artist, and artists evolve and change. As a rule of thumb stagnation is intellectual death.
I'm a big, big fan of Manson's early stuff. It's so angry and raw. After Golden Age of Grotesque his music took a downswing because the guy went through a string of bad relationships that destroyed his morale. The anger simply wasn't there. Like the three albums that came before it I didn't like The Pale Emperor, but when listening to it and tracks like Mephistopheles of Los Angeles I realized Manson was starting to get back to "it" - his out of the box thinking - even if the anger wasn't there.
I thoroughly enjoyed his last album Heaven Upside Down however. The anger isn't as intense as it was during his youth, but it's there. The album has this tone of smug dissatisfaction, reminding me of one of my other favourite bands as a youth - Garbage. Kill4Me could easily be sung by Shirley Manson and pass for one of their songs.
Criticizing Manson's music is one thing, but to criticize the man today because he doesn't superficially resemble the man he was yesterday is just silly. Manson is as Satanic as he ever was, whether or not his music is good. His slide from in-your-face shock rock into slower melodic music reminds me of how LaVey went from being a very public face for Satanism to a recluse shunning the spotlight. People change as their perspectives and desires also change.
That said there are very few artists that retain their integrity, fewer still that retain the essence that permeated their greatest works. When the fire dies it often is never reignited. Disturbed is the best example of this - The Sickness is STILL one of the best metal albums of all time, but everything they did afterward lacks the soul and fire of the first album. They may have a good track here and there, but Disturbed is just some nondistinct NuMetal band now. In contrast KoRn may have albums that are better and albums that are worse, but on the overall have never missed a single beat even when they made a dubstep album. Jonathan Davis is a passionate artist who loves what he does and it shows through his work. I feel a similar way about Rob Zombie.