I gave it three stars, because I'm not sure how to rate this item. I bought this from Amazon, opened it, and threw it in the car. I really just wanted it for those just-in-case moments, as I'm pretty good about keeping my phone charged at home. But, I tried it out a couple of weeks after I purchased it. In the 30 minutes that I used it to charge my iPhone 4, the iPhone got very hot. I mean, uncomfortably hot. Just to make sure, I charged it with the normal "sugar cube" charger that came with it, and also charged it using my laptop. In both cases, my iPhone didn't get much warmer than when you just leave it in your pocket and your body heats it up. Something was clearly wrong.
I called up Apple and explained the problem to them. I pointed out that the sugar cube charger was rated at 1 amp, but this Griffin charger was rated at 2.1 amps. Was it charging the phone a lot faster, and hence, generating more heat? They said that some chargers are designed to charge iPads, and maybe I should avoid those. In any case, they suggested that I discontinue using this particular charger.
Amazon was very gracious, and told me they would accept the return even though I had already thrown out the original packaging.
I'm giving it three stars, because clearly it works for other folks, just not me.
One other thing to note. In broad daylight, you cannot really see the green glow when this device is enabled--I was only able to see it in the garage. That's probably a good thing, otherwise the light from it might overwhelm your eyes when driving at night.