These servos use a three-pole motor geared *way* down. This gives great torque, but lousy resolution.
Lots of heli guys have tried these with predictable results - a machine that won't "lock-in" to a hover. Yuo can search the heli forums for more info.
That being said, I have a set of 615MGs running the tail of my Ultra Stick. They are slow to find absolute center, but are OK in this application as the plane is rarely going in a straight line.
