Remember to burn through your gasoline/ethanol blend on a regular basis. Fuel goes bad, and it is best for the engine to run regularly. I don't know what BMW does to mitigate any issues, if they run the engine regularly or not. By regular it might not be a bad idea to burn through your tank every month or two and keep it full when not using it.
I wouldn't get the REx and plan on never using it since it might not work when you do need it if that is the case.
Eric