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I purchased my in-car transmitter about 6 months ago from Halfords, at the time it cost me £30 but if you look at a number of different retailers you will find it's gone down in price since then (or maybe I didn't look around hard enough for mine!).
I bought it because I was fed up with my other transmitter which was only a cheap product, would lose connection and for a while it made me think my cigarette lighter in my car was funny, but anyway...
In the box came the transmitter, and a very bulky instructions book. The book is unnecessary and just looking at the product makes its use and set-up self explanatory. The product itself features the plug, which goes in the cigarette lighter, the headphone lead that allows you to use any iPod/mp3 player, and the transmitter. The transmitter has a screen that tells you which station it is currently linking to your car radio. The large (C) button on the device scans the area you are in and tells you which FM frequency is clear to use. You must then change your radio station manually to match this station for crystal clear listening. There are a number of other buttons on the device that allow you to manually change the station frequency up or down, and a 'save station' feature, which I don't use. I have found that if you go from one end of the city to the other the clearness of the frequency will change by a digit or two, meaning you will want to re-scan anyway making the save feature, in essence, redundant.
On top of the cigarette plug is a USB slot, allowing you to charge your iPod on the go as well. I'm not sure how much of an issue this could be on your car battery but I'm sure the car battery usage if minimal (with reference to in car phone chargers, car Bluetooth devices etc).
It is very easy to use and the wires don't get in the way of your gear stick/handbrake.
To date, the product hasn't let me down. I know that when I go for a drive, no matter how long, it's quick and easy to set up, takes seconds to find a clear station and transmits clear tunes. Much more reliable than my last device, good value for money, and would recommend it!