My personal opinion - I would not install one of these devices in my vehicle. As dexman has suggested, other people have complained about the device draining their battery. (why does this question and our responses seem like deja-vu?)
If you want a device which can read and clear odb2 codes, on Amazon and eBAY there are very inexpensive bluetooth enabled odb2 scanners that plug right into the socket the emission test places plug into. You download a odb2 scanner application from the PlayStore onto your phone (I have an Android) and go through the BT pairing process. I do not know what's available in iTunes because I don't have an iPhone.
I have used mine multiple times in a Jeep Wrangler, Honda Accord, and Audi Q4 to examine, research, and clear codes. If my check engine light goes on, I want to know why BEFORE bringing it to a repair shop. About 80% of the time, it's a code caused by a loose gas cap.
Note: if you are coming up for an emissions test and you clear codes, the emissions test places can detect this and fail your car.
I'd recommend one of these inexpensive odb2 bluetooth scanners above and beyond a HUM anytime.