I use these servos extensively. They are very good for mid-sized Park Models, about 45" to 60" span with weight less than 1.5 Kg. Carbon composite gears are better than nylon, about in the same league as Delrin gears, although Delrin has better self-lubrication properties,
They come complete with servo arms, mounting grommet, brass eyelet and screws.
Their price (in HK) is cheaper than HXT 900 - $ 2.6 vs. $ 2.9!
They weigh 17 gms bare, mounted will be upwards of 20. So obviously do not use them on smaller models and light foamies.
There is a heavier duty version too with Metal Gears, 19 gms, for $ 4.06 which I use for heavier applications:
http://www.hobbyking.com/hobbyking/store/__23486__HK15148B_Digital_Servo_19g_2_8kg_0_14s.html