SRAM X.9 10 Speed Rear Derailleur 2013
The new SRAM X9 10-speed rear derailleur brings you legendary SRAM X0 performance and reliability for less (and with only a slight weight penalty). It features Exact Actuation ratio technology for lightning-quick shift response and Di.R.T (Direct Route Technology) for top performance with minimal shifting effort. You can spend more if you want to, but if crisp, efficient gear changes are what you're after, X9 will suit you just fine.
This is the first time I have tried Sram for my drivetrain. After doing much reading and comparison shopping I went with the X9 model to match the rest of my stuff. So far it looks great, carbon outer cage and black.