I have been ridding the XChallenge like I stole it. Hard throttle, off road excursions, and constant 70-80mph cruising. I filled it Sunday and it took 3.13 gallons for exactly 180 miles (fill up to fill up). Overall I'm very satisfied, and it beats filling the Tundra.
This is one of the reasons I loved my F650GS (back when the 'F' version was the single-cylinder) - incredible mileage and it's loads of fun. The only thing I didn't like was that the engine felt strained over 70 mph (plenty of revs left, just didn't have the heart to make the engine work any more!).