Been using the Shadow m781 XT rear der for the last 6 months and I love it. Quick precise shifts. I have x9 on my other bike and I like the feel of the XT better. It feels smoother and seems to shift faster. And FYI, the other review was wrong about the clutch. There is no clutch on the m781.
I used one of these for 4 months. It is now in two pieces. It worked very well for about 2 months, and the Dyna-Sys clutch does work well. It does not work well on cassettes larger than 32 tooth, despite what shimano says. It quickly developed a lot of slop at every pivot, and the pulleys wore to their own unique shape. The cage is so thin, it will snap eventually if you ride hard. If you just xc ride, it is a solid unit. Don't put it on an all-mountain bike. Also, the cable guide rubs the cable on metal, so keep extra cables in your pack if you own this one. The older XT (9speed and earlier) are much better quality. Editor's Note: The M781 XT rear derailleur does not have the Shadow Plus clutch system mentioned in this customers review.