It should be OK for people living in cities or areas with good coverage.
$15 / month sounds more reasonable to me for people that spend a lot of time in their cars. I wouldn't use it enough to see the value in it.
All these different cars have different setups to access cool info. From the looks of it the Audi FORCES you to get the T-Mobile service in order to get the Google Maps and stuff. You can't just connect a phone's Data via Bluetooh and access that way. Kinda lame in that regard.