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 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.
Con Inconsistent quality
About 1/3 of reviews on Amazon mention the device stopped working or worked inconsistently after awhile.