MM is a very clever bloke. He's created a superb persona and image, the likes of which we've not seen since Bowie in the early 70's.
Everything he does is part of a calculated act, and his success is about more than his music, you have to buy into the Manson product.
Musically, it's not dreadful, but he's not particularly talented either. It lost its edge when he moved away from Trent Reznor producing his stuff. His initial stuff was just raw and poor musically, the Reznor produced tracks were edgy and musically coherent, then his work after creeping out from under Reznor's wings (Mechanical Animals and onwards), is just pop/pulp/rock. It's easier to listen to, and easier to sell as music to the public.
I like the odd track here and there, and think his cover of Sweet Dreams is excellent. However, it's mostly bland at best and I just don't buy into the product.
Still, can't knock the bloke for making a fortune out of himself
Edit-
Heresy by Trent Reznor said:
he sewed his eyes shut because he was afraid to see
he tries to tell me what i pit inside of me
he's got the answers to ease my curiosity
he dreamed a god up and called it christianity
you god is dead and no one cares
if there is a hell i will see you there
he flexed his muscles to keep his flock of sheep in line
he made a virus that would kill off all the swine
his perfect kingdom of killing, suffering and pain
demands devotion atrocities done in his name
you gos is dead and no one cares
drowning in his own hypocrisy
and if there is a hell i will see you there
burning with your god in humility
will you die for this?
Reznor was doing the anti-religion thing a long time before Manson, but gave up when Manson took up that mantle.
Manson is very similar to Microsoft. Inferior versions of everyone else's separate products, but packaged in a popular and sellable way.