I've used one a few times at race meetings. It makes the low end a lot more driveable - especially on low grip tracks. It also seems to protect my rotor (a Tekin - where I'd already broken two rotors!) better.
Overall, it makes the car easier to drive and more reliable, calms your throttle responses down too. Only issue is you lose some of the 'pop' that you need over jumps - though still more than ANY nitro - and this is on a loose setting.
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