This little box has only one purpose: to act as a remote for your iPhone's home button. The device itself is nothing remarkable, but the in-car workaround it enables is pretty slick.
First, the reason the workaround exists is simple. A few manufacturers, most notably GM and Honda, have been incorporating Apple's Siri Eyes Free in new models. It basically remaps a long press of a car's voice-recognition button to summon Siri, letting you talk to her through the car's built-in microphone and speakers. It works with anything Siri's good for that doesn't require looking at the phone.
The thing is, not everyone has a new car with Siri Eyes Free, not every new car comes with Eyes Free, and some of us are perfectly happy with what we're driving.
Enter Mobile Home, which allows you to long-press your way to Siri without fumbling for your phone. Setup is simple: pair phone with Bluetooth-capable car, pair Mobile Home with phone, clip it within reach, you're in business. Then a long press (or a short press followed by a long one if your phone is passcode-protected) and you're talking to your phone through your not-necessarily new car.
If you want to listen to and control music you'll need some sort of audio hookup—Bluetooth streaming, USB, an iPhone or iPod adapter. We tried it out in a variety of setups, including new cars with all the fancy connection options and an older model with only a Bluetooth phone and a 3.5mm audio jack. It works reliably, emulating Eyes Free with all of the strengths and the handful of weaknesses of Siri. Sometimes she's not available, sometimes she doesn't hear well over tire roar or wind noise, and sometimes she can't find that one song. In our experience she still outdoes the systems built into most cars.
The biggest difference between this and a phone/car using Siri Eyes Free is that Eyes Free locks certain things out, including anything with a visual response. Because Mobile Home reacts as though you're holding the phone, you can still get stuff like web search results. If you didn't know, that can be distracting to look at while driving. Solution: Have some self-control and don't look at your damn phone.
The other noticeable difference is price. Those new cars that include Eyes Free require you to buy a new car. Mobile Home is ; not cheap, but if you want to get the same basic feature set in an older or non-Siri-Eyes-Free-having new car, Mobile Home is a relative bargain. The other alternative is to keep your phone within reach and give the home button a squeeze. Just promise not to look at the screen, okay?