Reign in Blood says it all. That album is a masterpiece! So I don't doubt Rick Rubin's skills at all. Even if Justin Timberlake isn't your favourite music, does that make his production bad? I don't think so.
But yeah, this will be interesting. I for one prefered the pre-Bob Rock producing, and it seems Rick Rubin is more along the lines of pre-Black Album producers than Bob Rock was.
But yeah, Metallica are being pretty gay with the whole Rock Band thing but we'll see...... We'll see...