The only thing I have against Rubin's work with AC/DC is it seemed a bit treble-heavy. You still knew that Cliff Williams was playing the bass, but it didn't sound in the front of the mix to my old ears. Stiff Upper Lip was a better disc than Ballbreaker, IMHO - but I think that had more to do with Angus & the boys' songwriting.
If Metallica goes in with the same kind of songwriting that sank their last few albums - well, even with Rubin behind the boards, you can't make ice cream out of shit. I'm willing to give the benefit of the doubt and will probably try to preview it before I buy it. (Ah, if only Metallica would allow you to stream their music on a service like Rhapsody - so I could hear it before I dropped any coin and keep things legal.)