If you drive nice and are running the stock tune, with the Ecoboost motor and AWD, realistically about 16 around town, 22 on the highway. Driving with an aggressive tune loaded, and "enjoying it," it drops to around 14 around town and maybe 19 on the highway. But to drive a 4 door huge station wagon that outperforms most sports cars and luxury performance cars/suvs you see on a daily basis out there is a blast and is totally worth the drop in MPGs.
Not sure what the naturally aspirated Flex gets for mileage though.
I actually despised the Flex when we shopped, but one dealer made me sit in one (almost forcibly), a loaded Limited with Ecoboost (and AWD), so I sat, and fell in love with the inside. Then I drove it, we signed for it an hour later. This is coming from a 6'7" guy who has only bought full size SUVs or mid-size or larger P/Us in the past as my daily drivers. I'm still not a huge fan of the external "look," but everything else about it makes me love it. It does real well in the snow too. Only thing missing for me was the ground clearance for hunting and fishing, but I solved that by buying my wife a 4dr Wrangler.