I've got a grand canyon AL 7.9 29er if that's the type of thing you are looking at?
Had it since October, its supposed to be the winter bike and come summer the plan is for it to be the local XC bike (no big hills near me so woodsy singletrack and Swinley is the usual local riding). Can't say i have seen the carbon version in person, so i cant comment on the difference and whether its worth the price. It does look like the geometry is the same / very similar on the alu and carbon bikes.
I don't think i have had it long enough to comment on its full potential and this is my first 29er, as well as coming from generally less XC-focussed bikes. In short, i am still getting used to it but i wouldn't describe the ride as "ploughing through things", and "flickable" doesnt jump to mind either...! "Effective" is the word i would probably use just now, mates say it is noticeably quick to follow (compared to my other bikes). Honestly i dont think i have completely clicked with it just yet for more technical trails but i am hoping that I will once the mud tyres come off and the trails improve. No issues dealing with Canyon or the bike so far and it was great value when i got it, shame if the exchange rate has made them pricier.
EDIT: 6'2" here, riding a 19" with a 70mm stem (came with a 90), standard everything else.