I bought it after reading many reviews. I own 2 iphone 4. When you connect it a message pop ups saying: "This accesory is not optimized for this iPhone. You may experience noise caused by cellular interference and a decrease in cellular signal strenght", just bypass the message.
The charger does work and it charges fast!!!. I had a few car chargers before but they were extremely slow recharging the iphone 4. I haven't noticed any interference nor decrease in the signal while charging it. I am planning on buying another one.
Note: the message popups when you connect the cable to the phone, regardless if it goes to the computer or to the charger. The Apple store does have it listed as compatible with the iPhone 4 (Mfr. Part Number: GC23092).
Update: Bought a second one. My husband is thrilled how fast it charges his iphone. He pretty much drains the battery by the early afternoon, since he uses it a lot for calling. Best charger I ever bought. Will be able to use it with my iPad, when I get one lol.