Well I'm afraid I don't have a lot to compare it with, my previous bike was a 1st gen GT i-Drive, 36lbs. And I've only really been doing this for a year.
Anyway, so far I've ridden it once, up & down a fire trail (sheldon in los gatos, ca), but it's just great - climbs smoothly, no traction problems at all and with the front fork on the 80mm travel, no problems with the front end getting light. Didn't mess with the rear shock (RP3) at all. Got to the top and put the fork back to normal & came back down. This was pretty much as I remembered from the demo ride (there was a local Ellsworth demo day 11/19) - really stable coming down fast (for me), where with my old bike I'd be hanging on and praying. It feels much lighter than my old bike, and you can get nicely airborne over small bumps just by loading the shocks and jumping.
I need to play with the fork settings a bit more (Talas RLC, too many knobs to play with), and possibly try new tyres (2.4 vs 2.1).
The brakes are super powerful (juicy 7) and they are not even broken in yet, but then again I'm comparing it to my old v-brakes.
The XO shifters are fantastic - amazingly positive feel, no slack at all, changes down well even under moderate torque. I was originally going with an all XT build in a (mostly futile) attempt to keep the cost down, but I was sold on these (and the brakes) by the demo ride. I'm a sucker for buying things I can see/try rather than simply read about.
Hope to take it on some more challenging single track next weekend.