Even for XC, I defer to other brands where they're superior. Here's the breakdown on my XC bike of components in Shimano's playing field:

Shimano:
-- Front Hub (came stock but will probably switch to Mavic wheelsets in the future)
-- Rear Hub (came stock but will probably switch to Mavic wheelsets in the future)
-- Shifters
-- Brake Levers
-- Computer
-- Cables
-- Front Derailleur
-- Rear Derailleur
-- Cassette

Race Face:
-- Headset
-- Stem
-- Bottom Bracket
-- Crankarms
-- Chainrings

Avid:
-- Front Brake
-- Rear Brake

SRAM:
-- Chain

Pedals:
-- Time

Shoes:
-- Sidi

From past experience with Shimano's MTB stuff, I find that in general, they're pretty good with the intricate parts (Shimano shifting is uber smooth) but get surpassed in the areas where the parts are simpler but need to be more durable.