What speeds are we talking about here? I think that factors in when opening the windows.
From the revisited "A/C vs. Windows" myth/debate on MythBusters:
The fundamental flaw in the MythBusters' test was that the point where the drag becomes powerful enough to inhibit a car's performance with windows down was inside their 45–55mph margin at 50mph. Going less than 50mph it is more efficient to leave the windows down, but going greater than 50mph it is more efficient to use the A/C.
Personally, I don't open the windows too often, even if it's a nice day - and certainly not at highway speeds. I'll either use A/C (if needed) or nothing.