I disagree. My XT rear derailleur has been far more durable than my X.9 ever was. The X.9 main pivot bushing loosened up on me. My XT is still laterally tight. I'm also not a fan of the thumb triggers on the X.9. Not because I'm a moron and can't get used to them. I became accustomed to them in an hour. It was b/c of the where the thumb trigger resides. When set up to where I could reach both thumb levers comfortably...I would accidentally bump the trigger with my thumb on rough landings or rock gardens.

Still an opinion based on my experience and riding style. I have thought about going with X.0 twisters and RD...but I dunno if I can go back to twisters after using rapid fire 9 speed triggers. I'm also assuming that the X.0 RD won't accel in the durability dept. due to the carbon construction.