When Shimano released its "deer head" rear derailleur back in the early 80's, it revolutionized the cycling world by effectively creating the world's first mountain bike drivetrain. Ever since then, Deore XT has come to be known as the benchmark for MTB groups. For this particular model, Shimano updated its XT rear mech to work specifically with 10-speed drivetrains.
I purchased this derailleur in the spring, and have been abusing it on the road, trails, and in very rocky terrain. I've hit the derailleur multiple times on rocks and trees, to the point of breaking the hanger a month ago. With a new hanger put on, this derailleur still shifts as smooth and cleanly now as it did out of the box. I would recommend this to anyone in need of an affordable 10 speed derailleur.
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.