I recently purchased a 2012 Fiat 500c with with No Bluetooth. I wanted to stream music from my iPhone and use the phone hands free. Bluetooth receivers allowed for streamed music but not phone calls. Units like the Motorola Roadster are great for making calls but offer no solution for quality music.
The Calljax offers a brilliantly simple solution. It is basically an audio cable with a built in microphone. Think of it as turning your car stereo into a giant set of headphones. Hands free phone, stream music from any source, full Siri integration, voice enabled navigation. I know of no other device that can do all this, especially for 20 bucks.
The only issue I had was a volume imbalance between music, Siri, and the phone. A little adjustment and I was good to go. I can't speak for Android devices, but on an iPhone the Calljax is an incredible product.