this has been a big argument amungst my friends, on the value of grounding wires. Long story short, they do help, and i love it, because nobody believes it, and I love proving people wrong :-p
You know those times you are driving around, then you put on your air conditioning, your bright lights, and your stereio on all at the same time, and you can hear your car groaning? That's because you just put your car under a lot of electrical stress. Now, your car is always under stress, even when idling with no major componants running, but of course you don't notice that, because the car is in a "normal" state that it's created to run at. However, if you get rid of that stress by allowing your car to receive more power (ie more efficient grounds), then you get more power.
My eclipse used to run at exactly 12.7 volts in idle. Adding the grounds pushed it up to 12.9 when idle. Not that big of a difference, but it's well noticable when I put my car to the test at the track. Also, if you have an alternator pully installed on your car, the ground wires are a MUST.
Your headlights will become noticably brighter if you install the grounding wires correctly.
This is the first modification I've put on any car since learning about them. The benefits far outweight the cost...in fact, the gasmilage saved in a year will pay for them. (if you choose to buy them instead of make the kit yourself)
If you have a rx7, then most defintily add these. That was the car that Import Tuner got the best results in (12 HP gain at the highest point).
Anywhere, there you go guys