I just replaced the cheapo one in my ECX Circuit. Wouldn't even turn the wheels at a full stop, and slow response. I replaced it with this. An Arrma ADS-7 V2 7KG Waterproof Servo.
Now the wheels turn at a dead stop and much faster response. It has dual ball bearings, metal gears and won't break the bank. Got mine from Ebay for 19.00, but if you can find a hobby store carrying Arrma vehicles you should be able to get one. May have to order it, but I know it will fit and work well. Very happy with mine so far. Bashed it pretty hard 2 weeks ago. It's on the bench for repairs, but the servo is fine.