I got excellent mileage (60+ MPG US Gallons) on my trip from Columbus, MS to Nashville, TN a couple months back with my V outfitted as it is now (Trax bags fully loaded, skidplate, crash bars, radiator grill, handguards, and Powerbronze windhsield), although I did have a pretty constant speed of 55-60 MPH.
I'm sure all the metal I've added to the bike has decreased the MPG a little, but the looks are worth it! Every once in a while I'll take off the bags and their mounts (those things are a lot heavier than I thought) and go for a ride. The bike is always a lot more responsive and "flickable" in the corners. I really appreciate the utility of the side bags though and seldom have them off anymore.
My normal MPG ranges anywhere from 40 to 50 depending on how I ride.
2012 Kawasaki Versys 650 - 20,500 miles
2000 Volvo V70XC 2.4T - 258,000 miles