I am running a futaba s9351 servo in my mrx4x.I just started onroad last year and the servo is still working perfectly.I crashed the car left and right a million times and it still centers perfectly.Very tough servo and in hrs mode is awesome.I am using the s9551 servo for the throttle and brakes.It is low profile so I can stuff all my wires neatly under there.The s9551 buzzes alot trying to keep itself centered against the tension of the spring.I am running this off of a Team Orion Marathon 1600MAh 6 volt reciever pack and it last all day long no problem.Servo cost about 120 for the s9351 and the reciever pack 25.

s9351 specs:
Speed: 0.16 sec/60° @ 4.8V
0.13 sec/60° @ 6V
Torque: 153 oz-in @ 4.8V
192 oz-in @ 6V

s9551 SPECS:
Speed: 0.14 sec/60° @ 4.8V
0.11 sec/60° @ 6.0V
Torque: 98 oz-in (7.0kg/cm) @ 4.8V
122 oz-in (8.8 kg-cm) @ 6.0V