He's always a been a great bass player, and I think he fits in perfectly with the Metallica sound - particularly on Cyanide and My Apocalypse. Like the band point out in Some Kind Of Monster, as good a bass player as Newsted is it always annoyed me that he played with a pic, and like Cliff Burton, Trujillo plays with his fingers, which compliments the Metallica sound. But I'm a drummer so what do I know...
Metallica have been pretty anti bass players since Cliff died. Jason was awesome and even Bob Rock can play a bit, tho wasn't really required to during the Some Kind of Monster/St. Anger debacle.
Rob has a proven track record and the fact that he has co-writing credits on all tracks off Death Magnetic shows he's a genuine part of the band. He''s lost a bit of the funk but you wouldn't expect slap bass all over a Metallica album (!) and I think he's pretty tight with Lars' staccato riffs. True the album mix is pretty abysmal but the Guitar Hero III version is immense. The overall sound even on the quieter moments is distinctly heavy so i think the other three have warmed to having a quality bassist to play off against.
Hopefully Metallica will be fired up enough to produce another album of original material within 12 months rather than waiting another 5 years.