I've had several servos, several brands, in both airplanes, and cars, both on and off road. For racing, my favorites have been the JR high torque metal gear units. I've got a 650 in my mayhem, and another in my pro-4. Then 590s in a few other cars, and even a couple base models on throttles in a few others. They've all been great servos, only one or two going bad in rough situations. But I stand by them, they work very well, and won't break the bank...
If $$$ isn't an issue, the digis are really nice, but they suck a battery dry pretty quick. I see guys topping them off between races, and that's with 1000-1600 mah reciever packs. We race on a regular basis, and my analog servos hang with all the digi guys, no problem...