From the North East as well...My M5 is my DD - however I also have a Grand Cherokee for snow days. This year it paid off. Last year, however, I never needed to use the Grand Cherokee.
I drive to/from work (less than a mile each way)6 days a week and generally spend an additional 90 minutes or so driving for fun every morning. I'm averaging about 13.1 MPG but I'm always on sport plus modes (aside from suspension).
I'm happy with my M5 as a DD - but by all means get the 550. Only makes the rest of us look that much better!