It's a perpetual license, once off fee, for as long as you have the car. If you sell the car the subscription doesn't carry to your new car (if it's a BMW), you have to purchase again. I've done this twice now, on my M6 I purchased, and then again on the 760. Mine works via Bluetooth. You'll need to log into your car's iDrive with your BMW account, and it'll pull through CarPlay. If you're not logged in, then CarPlay will not reflect on your system when you're in the car.
In my opinion, worth it.