They're lightweight, and I liked the shape (though a bit small). Problems were the tiny traction pins, the fact that they don't thread in from the under side... and the spindles are pretty weak. Bent after a handful of rides.
Have had XT, XTR... all with satisfactory results. Got an X9 setup on my trail/freeride bike with a SRAM chain and it is pretty much awesome. I've had it on there for 3 years now and even still, when I need to pop quick shifts on Bobsled Trail, I know it's going to put the chain onto the next gear with precision. I always felt like the XT/XTR were a little sloppy when the chain was bouncing around a little. I love the stronger spring in the X9 as well. I say that this is the derailleur to get. X0 is supposed to be great... but not all of us have $200 to drop on something that could get ripped off by a random rock/root. :/
Love, love, love these shifters. My bike came with XTR rapid demise, and was the second thing to go (after the forks). The X9 is everything that XTR wasn't. Gunshot engagement, thumb friendly design.... you can even reach under with the index finger and pop a down shift if you so desire. They're attractive, simple, reliably built. Paired with an X9 and you've got the rear shifting of your freeride bike dialed.