I've never seen any instances where using the AC reduces MPG by more than 1-2 MPG so call me wasteful (money, fuel, environmentally-unfriendly), I'm using the AC. I've had to use the AC at 7am when it's only 65º F outside but the humidity was horrible.
Besides, I've found that most modern cars, at least those with any half-decent amount of aerodynamics, aren't enjoyable with the windows down, at least when you get over ~40 mph or so. The wind noise, the ear-thumping buffeting, and the turbulence just aren't nice.