There are a lot of devices on the market that do this sort of thing without wires, you know, those wireless FM transmitters that promise to push your iPod tunes to your car stereo, they work, but sometimes are a bit choppy. There are also cassette based solutions that do a hard-wired connection, just like this product, but you have to change tracks on your iPod still.
The Griffin Smartdeck is great if you have an older car, or one with a built-in unit that can't be changed. You simply plug in one end to your iPod (or any MP3 player with a 3.5mm jack) and then slot that cassette into your car stereo player. The thing that differentiates this product to cheaper offerings, is that after plugging in, you can hide your iPod away. The Smartdeck allows you to control the iPod tracks with the controls on your car's cassette player. I must admit it works very well indeed and allows you to change tracks.
Sound quality is very good. The fact that it is wired means you get no station interference, so the sound is solid. It is definitely one of the better solutions out there if you have an older car stereo that can take cassettes.