is it worth getting?

 

Probably a good idea for logging business mileage, but most may not need it or use the functions. Here are the details:

The optional electronic logbook enables automatic recording of mileage, route distance, date and time, starting point, and destination as part of every journey. Once you have dowloaded the logbooks from the PCM via Bluettoth or the USB interface, the data can be evaluted on your home PC using the supplied software.

You can order the car with it, or it's also available as a retrofit: 991 Log Book Retrofit