If you value a drivetrain that shifts consistently and a chain that won't rattle on rough terrain, look no further than the Shimano XTR M985 Shadow Plus rear derailleur. The revolutionary Shadow Plus switch adds tension to the derailleur pulley spring to nearly eliminate chain slap and ghost shifting.
Strengths: SMOOOOTH! This thing is truly like butter. I use the shadow in very bumpy rough conditions and turn it off when in smooth areas
Weaknesses: None, The only weakness is that I dont have this derailleur on every bike I own.
This derailleur came stock on my 13 specialized Stumpjumper FSR and I love it. Over 300 miles on the bike this year and not one complaint. Its smooth, consistent, and provides good feedback. Very quick shifting.