It's really early for me to say, as I've only tried them literally up and down the road to test the shifting. That said, it seems (to me at least) much, much snappier/quicker, with less travel on the levers, making for more comfortable shifting - IMO Deore feels a bit lazy in comparison, although it still does the job.
I'm sure there will be lots of people on STW who can give long-term advice on this. I know that one key difference surrounds the use of bearings in the XT version.
I'm going to try and get out tomorrow evening to try them out properly, but initial impressions are very favourable (- I've only used standard Deore before). Got them for £48 from Ribble (with an additional discount code) ... seems a pretty good price.