I'd like to contribute here with a few pictures and details to give an example of what I feel is a very well executed arm. (I really don't mean to be bragging, I just know it works really well.)
Let me start with:
KOP FP motor and transmission and an additional 4.125 chain reduction for a total reduction of ~495:1. With the efficiency of the stages added in we have a useful power reduction of 495*.59 = 292.5:1. (BTW, the floating chain tensioner has been replaced with a fixed one, as seen in the second picture.)
The surgical tubing was added to assist the motor. It balances the arm, when the arm is retracted, at about 120 deg. When the arm is extended, it balances at about 60 deg.
This arm can easily swing 270 degrees in 4 seconds. The thing to understand is this speed is with use limiting the drive to the FP motor to only +/-35%. We are using a PID control for both positioning and to holding the arm. Honestly, even if we remove power, the arm holds it's position.
The best part is, this motor barely, if ever, rises above room temperature.