Drove a rental GLA last September, and the system was quite nice to use. No noticeable lag.

Also, the integration with the iphone was top-notch. It understood that the iphone provided navigation, so if I pressed the nav button while on the radio screen, it would activate google maps from the iphone. The only thing which wasn't great was typing in some long address, which is why I connected the iphone.

Rented some kind of Audi a few weeks later, and it had the usual crappy integration. "nav" is the car's laggy navigation, etc.