LOL....Im just going off of ebay prices. XO and XX cassettes are going for ~250, I think. The XX is a drop 1/4 pound of so over the XT!wuzilla said:XO and XX cassettes the same price? Unless we're thinking of different things, you might want to double-check.
To answer your question, it would be stupid not to put an XX cassette on your bike. Beneficial? Meh. You may have to spend a few hours on a street corner to pay for it, but it would be stupid NOT to put something that pretty on your bike!