I ride a R1200GSA and have been using a Garmin Zumo 550 for several years. For road use, I have found it to be a great GPS. My Zumo is wired to the BMW power connector under the front beak.
I recently added a Cardo Scala Rider G4 Bluetooth communicator to my helmet and paired it with the Zumo for the audio GPS turn instructions, and paired the Zumo with my Android phone so I can receive phone calls. A Garmin nuMaps lifetime subscription is a worthwhile addition. However, the Zumo 550 now lacks sufficient memory to load all the U.S. and Canada maps in the unit's memory. You can load the maps on an SD card.
Combined with an SD card, the Zumo 550 can store as many data files with routes, waypoints, POIs, etc. as you want, along with MP3 music. I even created some "pictures" files with text information on how to do the really obscure, infrequently performed operations for the GPS.
Others have indicated the Zumo is not as well suited for offroad riding because it lacks good support for tracks. I am now considering a Garmin Montana for similar reasons.
A buddy of mine just added a Garmin Nuvi 550 to his R1200GSA. He indicates it is waterproof. This could be a good, less expensive alternative to a Zumo 550 or Montana.
2015 BMW R1200RT, 2016 BMW S1000XR