I installed a HS-525-BB and an HS-645-MG servo in my kyosho buggy.
The HS-525-BB is great value for money, I notices much crisper throttle response, going from full throttle to full brake is much less delayed, and going from full brake to full throttle is much faster also.
Making adjustments to your vehicle pitch whilst airborne is also much quicker...
I am now on my second HS-525-BB throttle servo, becuase I burnt one out.
The difference you will notice will depend on previous servo, so you should notice some difference.