The XO rear derailleur has been re-designed to fit modern mountain drivetrains that require functionality with 2x10 cranksets and larger 36t cassettes as well as tightened up geometry for narrower 10 speed chains. The cage has been beefed up for stiffness and added shifting precision. The stronger return spring helps the derailleur travel back and forth with ease lessening the load on the shifters. The cage features an alloy inner plate with a carbon outer plate for some weight reduction.
SRAM took the time and effort to make the XO a quality component, with tight tolerances and putting a quality finish on even the smallest items, such as the fixing and adjustment screws. If it wasn't for XX, this derailleur could easily serve as a top offering in the SRAM line-up. Light enough for a XC race steed but plenty durable for the rigors of all mountain abuse, the XO does it all. Select medium length cage for 2x10 or long for a triple crankset.
X0 replaced SRAM X3.0 rear derailleur on my Bike Friday touring bicycle. Much better shifting with wide range cassettes. Extremely crisp shifting with my close ratio "commuting" cassette. Ive had one fall, fully laden, on the derailleur side with some cosmetic scuffing of the aluminium but function is still spot on. Ive used Shimano XT, Ultegra, Dura Ace and Chorus. The X0 is up there with the best.