As requested, here is a clip of one of my test flights with the APM2 on my 600. This may be the most boring video ever, but in this case, boring is exciting! The APM2 works amazingly well mounted on Align gel, where a tail gyro normally sits. Stabilization is very good using these parameters, though I need to lower the yaw values since there's still just a bit of tail wagging.
I've had this bird for a couple years, so unfortunately I think there is a bit of play in the linkages, and a couple of the servos have sticky spots. This causes small variations in the rotor disc (leading to slight wandering) which the APM is not responsible for.
The previous stabilization system I had was a Robbe HeliCommand. Absolute rubbish. That caused more grief and wasted time than I can imagine. Although, I was terrified of tuning it and knew very little about the effects of vibrations, so maybe it was good and I didn't know it!
Either way, APM2 + TRex is awesome so far.