It's crap. Even the stereo caps that were eluded to above are crap! Completely worthless.
They are designed to store energy and release a charge when the voltage gets down to a certain level, i.e. a strong bass hit on a subwoofer. You may think to yourself, "Well, that sounds like a good thing!" Let me explain a few things...
Capacitors store up energy.
They release it quickly.
Then they need to be charged again.
If you've ever read the instructions that come with one of these, they usually recommend "slow" charging it using a resistor for an hour or so after initial installation BEFORE using it. Even after it's charged and ready for use, it still takes several seconds to a few minutes to fully recharge after a hard bass hit.
So where does that energy come from? Your car's electrical system. If you listen to a song that only has a hard bass hit once every few seconds, (depending on the cap) you can get away with it. Of course, the majority of songs that you want to crank up at midnight to annoy the neighbors have a few bass hits per second. Even a "good" cap can only keep up for a few beats and then you are back to relying on your cars constant voltage output.
The people that say it makes a difference, probably just ended up inadvertently improving their car's electrical system. This is what the Pro's do in the first place (no cap involved) and it's the least expensive upgrade (that actually works) to do.. It's called the Big 3:
1) Increase the gauge of the battery's grounding wire.
2) Increase the gauge of the alternator's grounding wire.
3) Increase the gauge of the alternator's charging wire.
All three of these reduce resistance of the electrical flow throughout the entire vehicle. More flow = Faster battery charging.
Now mind you, this only works to a point. There are some other things you can do if you need more power:
1) Get a better battery, Optima yellow-tops are the standard, but there are other choices out there.
2) Add a second battery. (Just make sure they are hooked up correctly. Don't end up make a 24v system out of your 12v one).
3) Add a high output alternator. You can get really crazy and even add a second one.
After reading this, you may say that guy is full of crap. Believe what you want, this is the internet after all, but here's my experience on the matter:
I had a pretty decent system in my SUV. I was getting quite a bit of dimming. I went to the local ripoff shop and they recommended a cap. I installed it according to directions and got know difference what so ever. I did some research on an online audio forum (much like this one, only more attitude) and found out about the Big 3. I removed the cap and tried the Big 3 and it actually worked.
I said, hmmm, maybe now I can go bigger and better with the system and added some more bass. It was still going strong, so I kept adding a little here and there. I eventually added an Optima battery and increased the gauges on the Big 3 again. I was up to 1300w RMS (about 3500-4000w peak) and only had minimal dimming. Granted I think it was more in my head from almost blacking out every time the bass hit! LOL!!!
Along came the Solstice and suddenly I had other things to spend your tax dollars (I mean my paycheck) on. I was going to add another battery and a HO alternator, but ended up giving the SUV to my father...