I have been reading about this on several other boards. The consensus is that they help by keeping the electrical system cleaner. The spark is more effective. They do not add anything. They reduce the effects of loss of current through poor grounding. Reports from people who have actually used them say they do not reduce any high power stereo loads or make the car feel any different. I would equate the using of these systems to getting bigger spark plug wires or a bigger gauge wire to ground the battery when installing a stereo. Helpful but probably not very noticable if you already have installed aftermarket plug wires. At their price ~$100 I don't think I will need them.
I've heard some great things about these kits. I personally have one hell of a grounding issue in my Celica and while I bet that would help the case, I don't care to spend the money just to sell it here in a few months.
They help the electrical system, if your factory grounds are painted over/corroded/not tight/etc etc. then the signals being sent to the ecu can be off. Adding extra ground wires makes sure that there are no faulty signals due to a bad ground. All of the sensors on your motor measure certain voltage ranges (ie.front O2 sensor 0.1v-5v), if the ground is intermittent or corroded(read higher resistance) then the ecu might not be seeing the true situation(it may see .8v instead of 1v). Doesn't sound that major to most people but when that is all the ecu has to rely on it can be bad over time.
Would I pay over $25 for one of those kits?? No.
Do they increase HP or Torque? No.
Do they help smooth out rough idle?? Yes if faulty grounds were the culprit.
Is it a good idea?? Yes since most factory ground spots are painted over and it could/will not harm your car.
Do they improve throttle response? Nominal if any, although the feeling of a smoother running engine might trick your mind to believe so.
With all of that said if you want Motorola spec ground straps for a fair price get in touch with me. They are flat braid copper tinned straps, from the leading experts in the electronics field.