A combination of hardware and app that supports iPhone and Android, different versions for Android only, iPhone only, and Android+iPhone with special versions for VW and BMW.
Ranked in these QuestionsQuestion Ranking
Pro Moderate price
Not the cheapest, but about $60.
Works with apps from different developers.
Con Inconsistent quality
About 1/3 of reviews on Amazon mention the device stopped working or worked inconsistently after awhile.
Con At least 5 different versions
Could be confusing to figure out specifically which version will work with which car and phone so it might be possible to accidentally order the wrong one.