The adapter plug is for the price more than perfect !! It is very stable, does not wobble and delivers perfectly electricity. We have an iPhone 4 and an iPhone 5S, both to be charged without a peep and very smoothly via USB while also the Navi is receiving power. We have never had an error message, which is not always self-evident for iPhone owners :-)
My conclusion: price / performance simply TOP !!! Highly recommended, especially for iPhone owners.