I've got the lastest XT's on one bike, had elixirs before that and XT-mini's before that.
The X2's are great for XC and plenty powerfull enough. Not as powerfull as the others, but close enough. I run them with lightweight rotors which dulls their power a bit and they're still good.
The XT's I thought were very good, but once set up how I liked them they're not as solid and powerfull as I rememeber, still more than good enough. Seem's they get worse the closer you run them to the bar (I run mine so I can pull the lever right to the bar and be only just locking up when braking with one wheel and my weight right off the back on the road).
The Elixirs were functionaly the best, although their reliability was lacking. More power than the XT's but that was useless if on one in 4 rides they'd pull to the bar!